/* ============================================================
   GIS & Remote Sensing Study Guide – Professional Book Stylesheet
   ============================================================ */

/* — Google Fonts import — */

:root {
  /* — Colour palette (B&W) — */
  --paper:       #ffffff;
  --paper-edge:  #f5f5f5;
  --ink:         #111111;
  --muted:       #666666;
  --accent:      #1a3c5e;
  --accent-light:#eef3f8;
  --accent-2:    #8b2500;
  --accent-2-lt: #fdf5f3;
  --panel:       var(--paper);
  --line:        #d0d0d0;
  --soft:        #f0f0f0;
  --gold:        #c9a84c;
  --green:       #2d6a4f;
  --green-lt:    #edf7f0;
  --page-w:      250mm;
  --page-min:    680px;
  --gutter:      64px;
  --radius:      8px;
  --shadow-sm:   0 2px 8px rgba(0,0,0,.06);
  --shadow-md:   0 4px 20px rgba(0,0,0,.10);
  --shadow-lg:   0 8px 40px rgba(0,0,0,.14);

  /* — Font sizes (uniform) — */
  --fs-body:     13.5pt;
  --fs-h1:       13.5pt;
  --fs-h2:       13.5pt;
  --fs-h3:       13.5pt;
  --fs-h4:       13.5pt;
  --fs-small:    13.5pt;
  --fs-tiny:     13.5pt;
  --fs-formula:  13.5pt;

  /* — SVG diagram palette — */
  --svg-bg:      #f0f4f8;
  --svg-card:    #ffffff;
  --svg-text:    #1e293b;
  --svg-sub:     #64748b;
  --svg-line:    #94a3b8;
  --svg-hub:     #2c5f8a;
  --svg-hub-t:   #ffffff;
  --svg-blue:    #dbeafe;
  --svg-blue-s:  #60a5fa;
  --svg-green:   #dcfce7;
  --svg-green-s: #34d399;
  --svg-yellow:  #fef3c7;
  --svg-yellow-s:#f59e0b;
  --svg-red:     #fee2e2;
  --svg-red-s:   #f87171;
  --svg-purple:  #e0e7ff;
  --svg-purple-s:#818cf8;
  --svg-pink:    #fce7f3;
  --svg-pink-s:  #f472b6;
  --svg-orange:  #ffedd5;
  --svg-orange-s:#fb923c;
  --svg-teal:    #ccfbf1;
  --svg-teal-s:  #2dd4bf;
  --svg-text-blue:   #1e40af;
  --svg-text-green:  #166534;
  --svg-text-red:    #991b1b;
  --svg-text-yellow: #92400e;
  --svg-text-orange: #9a3412;
  --svg-text-purple: #3730a3;
  --svg-text-pink:   #9d174d;
  --svg-text-teal:   #115e59;

  /* — Transitions — */
  --ease:        cubic-bezier(.4, 0, .2, 1);
  --dur:         .3s;
}

/* ---- Dark Mode ---- */
[data-theme="dark"] {
  --paper:       #1a1d23;
  --paper-edge:  #22262e;
  --ink:         #e0ddd5;
  --muted:       #9a9a9a;
  --accent:      #5ea3e0;
  --accent-light:#1e2a38;
  --accent-2:    #e8a090;
  --accent-2-lt: #2a2225;
  --line:        #3a3d44;
  --soft:        #24272e;
  --gold:        #d4b85c;
  --green:       #68c090;
  --green-lt:    #1e2a25;

  /* — SVG dark palette — */
  --svg-bg:      #1e2330;
  --svg-card:    #252a36;
  --svg-text:    #e2e8f0;
  --svg-sub:     #94a3b8;
  --svg-line:    #475569;
  --svg-hub:     #3b82f6;
  --svg-hub-t:   #ffffff;
  --svg-blue:    #1e3a5f;
  --svg-blue-s:  #3b82f6;
  --svg-green:   #14412a;
  --svg-green-s: #22c55e;
  --svg-yellow:  #422d08;
  --svg-yellow-s:#eab308;
  --svg-red:     #450a0a;
  --svg-red-s:   #ef4444;
  --svg-purple:  #2e1065;
  --svg-purple-s:#a78bfa;
  --svg-pink:    #4a1942;
  --svg-pink-s:  #ec4899;
  --svg-orange:  #431407;
  --svg-orange-s:#f97316;
  --svg-teal:    #0d3331;
  --svg-teal-s:  #14b8a6;
  --svg-text-blue:   #93c5fd;
  --svg-text-green:  #86efac;
  --svg-text-red:    #fca5a5;
  --svg-text-yellow: #fde68a;
  --svg-text-orange: #fdba74;
  --svg-text-purple: #c4b5fd;
  --svg-text-pink:   #f9a8d4;
  --svg-text-teal:   #5eead4;
}
[data-theme="dark"] body {
  background: linear-gradient(135deg, #111318 0%, #181b22 50%, #111318 100%);
}
[data-theme="dark"] .page {
  box-shadow: 0 4px 30px rgba(0,0,0,.35), 0 1px 4px rgba(0,0,0,.2);
  background-image: none;
}
[data-theme="dark"] .page::before {
  background: linear-gradient(180deg, #333 0%, #555 100%);
}
[data-theme="dark"] th {
  background: #2a2d33;
}
[data-theme="dark"] table {
  background: var(--paper);
}
[data-theme="dark"] .figure {
  background: var(--paper-edge);
  border-color: var(--line);
}
[data-theme="dark"] .figure svg {
  background: none;
}
[data-theme="dark"] ::selection {
  background: rgba(0,0,0,.12);
}
[data-theme="dark"] main h2,
[data-theme="dark"] main h3 {
  color: #e8805a;
  font-style: italic;
}
[data-theme="dark"] .meta-badge.exam {
  background: #ffffff;
  color: #000000;
  border: 1px solid #cccccc;
}
[data-theme="dark"] .exam-ref {
  background: #ffffff;
  color: #000000;
}
[data-theme="dark"] .exam-ref:hover {
  background: #e0e0e0;
  color: #000000;
}

/* ---- Smooth Theme Transition ---- */
html {
  overflow-x: hidden;
  transition: background-color .4s var(--ease);
}

/* ---- Custom Scrollbar ---- */
::-webkit-scrollbar { width: 7px; height: 7px; }
::-webkit-scrollbar-track { background: transparent; }
::-webkit-scrollbar-thumb {
  background: var(--line);
  border-radius: 4px;
}
::-webkit-scrollbar-thumb:hover { background: var(--muted); }
@supports (scrollbar-color: auto) {
  * { scrollbar-width: thin; scrollbar-color: var(--line) transparent; }
}

/* ---- Animations ---- */
@keyframes fadeInUp {
  from { opacity: 0; transform: translateY(24px); }
  to   { opacity: 1; transform: translateY(0); }
}
@keyframes fadeIn {
  from { opacity: 0; }
  to   { opacity: 1; }
}
@keyframes slideInLeft {
  from { opacity: 0; transform: translateX(-20px); }
  to   { opacity: 1; transform: translateX(0); }
}
@keyframes pulse {
  0%, 100% { transform: scale(1); }
  50%      { transform: scale(1.05); }
}
@keyframes gradientShift {
  0%   { background-position: 0% 50%; }
  50%  { background-position: 100% 50%; }
  100% { background-position: 0% 50%; }
}

/* ---- Reset ---- */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
html { scroll-behavior: smooth; }

/* ---- Body / Background ---- */
body {
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-body);
  color: var(--ink);
  background: var(--paper);
  line-height: 1.8;
  -webkit-font-smoothing: antialiased;
  transition: background var(--dur) var(--ease), color var(--dur) var(--ease);
}

/* ---- Scroll Progress Bar ---- */
.scroll-progress {
  position: fixed;
  top: 0;
  left: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--accent), var(--gold), var(--accent-2));
  background-size: 200% 100%;
  animation: gradientShift 3s ease infinite;
  z-index: 9999;
  transition: width .1s linear;
  border-radius: 0 2px 2px 0;
}

/* ---- Theme Toggle Button ---- */
.theme-toggle {
  position: fixed;
  top: 16px;
  right: 16px;
  z-index: 9998;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  border: 1.5px solid var(--line);
  background: var(--paper);
  color: var(--ink);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 20px;
  box-shadow: var(--shadow-sm);
  transition: all var(--dur) var(--ease);
  backdrop-filter: blur(8px);
}
.theme-toggle:hover {
  transform: rotate(30deg) scale(1.1);
  box-shadow: var(--shadow-md);
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

/* ---- Scroll-to-Top Button ---- */
.scroll-top {
  position: fixed;
  bottom: 24px;
  right: 16px;
  z-index: 9998;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  border: 1.5px solid var(--line);
  background: var(--paper);
  color: var(--accent);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 22px;
  box-shadow: var(--shadow-sm);
  transition: all var(--dur) var(--ease);
  opacity: 0;
  pointer-events: none;
  transform: translateY(12px);
}
.scroll-top.visible {
  opacity: 1;
  pointer-events: auto;
  transform: translateY(0);
}
.scroll-top:hover {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
  box-shadow: var(--shadow-md);
  transform: translateY(-2px);
}

/* ---- Page (A4-like card — realistic book page) ---- */
.page {
  width: min(var(--page-w), calc(100% - 32px));
  margin: 48px auto;
  background: var(--paper);
  border: none;
  border-radius: 1px 6px 6px 1px;
  box-shadow:
    -4px  0  0  -1px #e0e0e0,
    -8px  0  0  -2px #d5d5d5,
    -12px 0  0  -3px #cccccc,
    0 4px 24px rgba(0,0,0,.12),
    0 1px  4px rgba(0,0,0,.08);
  position: relative;
  counter-reset: page-section;
  background-image:
    repeating-linear-gradient(
      0deg,
      transparent,
      transparent 28px,
      rgba(0,0,0,.015) 28px,
      rgba(0,0,0,.015) 29px
    );
  background-color: var(--paper);
  animation: fadeInUp .6s var(--ease) both;
  transition: background var(--dur) var(--ease), box-shadow var(--dur) var(--ease);
}

/* Left-side spine accent — removed */
.page::before {
  display: none;
}

/* ---- Header (Chapter title band) ---- */
header {
  padding: 44px var(--gutter) 28px;
  border-bottom: none;
  position: relative;
  background:
    linear-gradient(180deg, rgba(0,0,0,.03) 0%, transparent 100%);
}

/* Decorative double-rule under header */
header::after {
  content: "";
  display: block;
  margin-top: 22px;
  height: 0;
  border-top: 2.5px solid var(--ink);
  border-bottom: .75px solid var(--line);
  padding-bottom: 3px;
}

.eyebrow {
  text-transform: uppercase;
  letter-spacing: .3em;
  font-size: var(--fs-small);
  color: var(--gold);
  margin-bottom: 4px;
  font-weight: 600;
  font-family: 'Times New Roman', Times, serif;
}

/* ---- Headings ---- */
h1, h2, h3, h4 {
  line-height: 1.25;
  color: var(--ink);
  font-family: 'Times New Roman', Times, serif;
  scroll-margin-top: 24px;
}

h1 {
  font-size: var(--fs-h1);
  font-weight: 800;
  margin-bottom: 6px;
  letter-spacing: -.02em;
}

h2 {
  font-size: var(--fs-h2);
  font-weight: 700;
  margin-top: 44px;
  margin-bottom: 18px;
  padding-bottom: 8px;
  border-bottom: none;
  position: relative;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: .04em;
}

/* Elegant gradient underline for h2 */
h2::after {
  content: "";
  display: block;
  margin-top: 6px;
  height: 2px;
  background: linear-gradient(90deg, var(--accent) 0%, var(--gold) 40%, transparent 100%);
  border-radius: 1px;
}

h3 {
  font-size: var(--fs-h3);
  font-weight: 700;
  margin-top: 32px;
  margin-bottom: 14px;
  color: var(--accent);
  padding-left: 14px;
  border-left: 3px solid var(--gold);
}

h4 {
  font-size: var(--fs-h4);
  font-weight: 700;
  margin-top: 24px;
  margin-bottom: 8px;
  color: var(--accent-2);
  font-style: italic;
}

/* ---- Body text ---- */
p, li, dd, dt {
  font-size: var(--fs-body);
  text-align: justify;
  hyphens: auto;
}

/* First-line indent for consecutive paragraphs (book convention) */
p + p {
  text-indent: 1.5em;
}

p  { margin: 0 0 14px; }
li { margin-bottom: 8px; }

strong { font-weight: 700; }

/* ---- Drop cap on first paragraph of lead text only ---- */
.drop-cap::first-letter {
  float: left;
  font-family: 'Times New Roman', Times, serif;
  font-size: 3.2em;
  line-height: .82;
  padding-right: 6px;
  padding-top: 4px;
  color: var(--accent);
  font-weight: 700;
}

/* ---- Main content area ---- */
main {
  padding: 32px var(--gutter) 56px;
}

.lead {
  font-size: var(--fs-body);
  color: var(--muted);
  max-width: 80ch;
  margin-bottom: 18px;
  font-style: italic;
}

/* ---- Links ---- */
a {
  color: var(--accent);
  text-decoration: none;
  border-bottom: .5px solid transparent;
  transition: border-color .2s;
}
a:hover {
  border-bottom-color: var(--accent);
  text-decoration: none;
}

/* ---- Focus states (accessibility) ---- */
:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
  border-radius: 2px;
}
.theme-toggle:focus-visible,
.scroll-top:focus-visible {
  outline-offset: 3px;
}
a:focus-visible {
  border-bottom-color: var(--accent);
  outline-offset: 1px;
}

/* ---- Lists ---- */
ul, ol { padding-left: 28px; margin: 10px 0 16px; }
ul ul, ol ol, ul ol, ol ul { margin-top: 6px; margin-bottom: 6px; }

/* Subtle custom square bullets */
ul > li { list-style-type: "\25AA\0020"; }
ul > li > ul > li { list-style-type: "\25AB\0020"; }

/* ---- Definition lists ---- */
dl { margin: 12px 0 18px; }
dt {
  font-weight: 700;
  color: var(--accent);
  margin-top: 10px;
}
dd {
  margin-left: 24px;
  margin-bottom: 6px;
}

/* ---- Blockquote ---- */
blockquote {
  margin: 18px 0;
  padding: 14px 20px 14px 24px;
  border-left: 3px solid var(--gold);
  background: var(--soft);
  border-radius: 0 var(--radius) var(--radius) 0;
  font-style: italic;
  color: var(--muted);
}

/* ---- Table of Contents ---- */
nav.toc {
  margin-top: 24px;
  padding: 20px 24px;
  border: 1pt solid var(--line);
  background: var(--paper-edge);
  border-radius: 3px;
}
nav.toc h2 { margin-top: 0; font-size: var(--fs-h3); }
nav.toc h2::after { display: none; }
nav.toc h2 { text-transform: none; letter-spacing: normal; }
nav.toc ol { columns: 2; column-gap: 36px; padding-left: 22px; margin: 0; }
nav.toc li { margin: 3px 0; break-inside: avoid; font-size: var(--fs-body); list-style-type: decimal; }

/* ---- Exam-note boxes (past questions per section) ---- */
.exam-note {
  border: 1px solid var(--line);
  border-left: 3.5pt solid var(--gold);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 18px 22px;
  margin: 18px 0 28px;
  background: var(--soft);
  break-inside: avoid;
  transition: border-color var(--dur) var(--ease);
}
.exam-note h4 {
  margin: 0 0 10px;
  padding-bottom: 8px;
  border-bottom: 1px dashed var(--line);
  color: var(--gold);
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-small);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: .06em;
}
.exam-note ol {
  padding-left: 22px;
  margin: 0;
}
.exam-note li {
  margin-bottom: 6px;
  font-size: var(--fs-body);
  line-height: 1.6;
}
.exam-note li em {
  color: var(--muted);
  font-size: var(--fs-small);
}
[data-theme="dark"] .exam-note {
  border-color: var(--line);
  border-left-color: var(--gold);
  background: var(--paper-edge);
}

/* ---- Important Things to Consider ---- */
.important-header {
  background: linear-gradient(135deg, #ef4444 0%, #dc2626 100%);
  color: #fff !important;
  padding: 14px 22px;
  border-radius: var(--radius, 8px);
  margin-top: 48px;
  font-size: 1.3em;
  letter-spacing: .02em;
}
.important-header + .card {
  margin-top: 0;
  border-radius: 0 0 var(--radius, 8px) var(--radius, 8px);
}
.important-header + .card h3 {
  color: #dc2626;
  border-bottom: 1px solid #fecaca;
  padding-bottom: 6px;
  margin-top: 22px;
}
.important-header + .card h3:first-child { margin-top: 0; }
[data-theme="dark"] .important-header {
  background: linear-gradient(135deg, #b91c1c 0%, #991b1b 100%);
}
[data-theme="dark"] .important-header + .card {
  background: #1e1e1e;
  border-left-color: #b91c1c;
}
[data-theme="dark"] .important-header + .card h3 {
  color: #fca5a5;
  border-bottom-color: #7f1d1d;
}

/* ---- Year Tags on Headings ---- */
.year-tag {
  display: inline-block;
  font-size: 0.65em;
  font-weight: 700;
  color: #fff;
  background: var(--accent);
  padding: 1px 7px;
  border-radius: 3px;
  margin-left: 5px;
  vertical-align: middle;
  letter-spacing: 0.02em;
  white-space: nowrap;
  line-height: 1.6;
}
.year-tag + .year-tag { margin-left: 3px; }
[data-theme="dark"] .year-tag {
  background: var(--accent);
  color: #fff;
}

/* ---- Callouts / Formula / Exam boxes ---- */
.callout, .formula, .exam-box {
  padding: 18px 22px 18px 50px;
  margin: 22px 0;
  border-left: 3.5pt solid var(--accent);
  background: var(--accent-light);
  border-radius: 0 var(--radius) var(--radius) 0;
  font-size: var(--fs-body);
  position: relative;
  transition: box-shadow var(--dur) var(--ease), transform var(--dur) var(--ease);
}

/* Callout icon */
.callout::before {
  content: "\2731";
  position: absolute;
  left: 14px;
  top: 14px;
  font-size: 16pt;
  line-height: 1;
}

.exam-box {
  border-left-color: var(--accent-2);
  background: var(--accent-2-lt);
}
.exam-box::before {
  content: "\270E";
}

.formula {
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-formula);
  background: var(--soft);
  text-align: center;
  padding: 20px 22px 20px 50px;
  line-height: 1.9;
  border-left-color: var(--gold);
}
.formula::before {
  content: "\2211";
}

/* ---- Two-column layout ---- */
.two-col { display: grid; grid-template-columns: repeat(2, minmax(0,1fr)); gap: 24px; }

/* ---- Tables ---- */
.table-wrap { overflow-x: auto; margin: 22px 0; border-radius: 4px; }

table {
  width: 100%;
  border-collapse: collapse;
  background: var(--paper);
  font-size: var(--fs-body);
  border: 1px solid var(--line);
  border-radius: 4px;
  overflow: hidden;
}
caption {
  caption-side: bottom;
  padding: 8px 0 0;
  font-style: italic;
  color: var(--muted);
  font-size: var(--fs-small);
}

th, td {
  border: .5pt solid var(--line);
  padding: 10px 14px;
  vertical-align: top;
  text-align: left;
}

th {
  background: var(--accent);
  color: white;
  font-weight: 700;
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-small);
  text-transform: uppercase;
  letter-spacing: .04em;
}

/* Alternating row stripe */
tbody tr:nth-child(even) {
  background: var(--paper-edge);
}
tbody tr:hover {
  background: var(--accent-light);
}

/* ---- Tags ---- */
.chapter-tag {
  display: inline-block;
  padding: 3px 10px;
  border: .5pt solid var(--line);
  background: var(--soft);
  font-size: var(--fs-small);
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--muted);
  margin-bottom: 12px;
  border-radius: 2px;
}

/* (exam-ref moved to enhanced section below) */

/* ---- Footer ---- */
.footer-note {
  margin-top: 48px;
  padding-top: 18px;
  border-top: none;
  font-style: italic;
  font-size: var(--fs-small);
  color: var(--muted);
  text-align: center;
  position: relative;
}

/* Ornamental rule above footer */
.footer-note::before {
  content: "\2014\00a0\00a7\00a0\2014";          /* — § — */
  display: block;
  text-align: center;
  color: var(--line);
  font-style: normal;
  font-size: 11pt;
  letter-spacing: .2em;
  margin-bottom: 10px;
  position: absolute;
  top: -6px;
  left: 50%;
  transform: translateX(-50%);
  background: var(--paper);
  padding: 0 16px;
}

.footer-note::after {
  content: "";
  position: absolute;
  top: 0;
  left: 15%;
  right: 15%;
  height: .5px;
  background: var(--line);
}

/* ---- Back-to-index link ---- */
.back-link {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  margin-bottom: 16px;
  padding: 6px 16px;
  border: 1px solid var(--line);
  background: var(--soft);
  font-size: var(--fs-small);
  font-family: 'Times New Roman', Times, serif;
  color: var(--accent);
  text-decoration: none;
  border-radius: 3px;
  transition: all .2s;
}
.back-link:hover {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

/* ============================================================
   Figures & Diagrams
   ============================================================ */
.figure {
  margin: 36px 0;
  padding: 24px 20px 16px;
  text-align: center;
  background:
    linear-gradient(180deg, #fff 0%, var(--paper) 100%);
  border: 1px solid var(--line);
  border-radius: 6px;
  box-shadow:
    0 2px 8px rgba(0,0,0,.05),
    0 0 0 1px rgba(0,0,0,.02);
  position: relative;
  transition: box-shadow .25s ease;
}

/* Reset default browser margins on nested <figure> inside .figure div */
.figure figure {
  margin: 0;
}

.figure:hover {
  box-shadow:
    0 4px 16px rgba(0,0,0,.08),
    0 0 0 1px rgba(0,0,0,.03);
}

/* Subtle figure number accent bar at top */
.figure::before {
  content: "";
  position: absolute;
  top: 0; left: 20%; right: 20%;
  height: 2px;
  background: linear-gradient(90deg, transparent, var(--gold), transparent);
  border-radius: 0 0 2px 2px;
}

.figure svg {
  max-width: 100%;
  height: auto;
  display: block;
  margin: 0 auto;
}

.figure figcaption {
  font-size: var(--fs-small);
  color: var(--muted);
  font-style: italic;
  margin-top: 14px;
  padding-top: 10px;
  text-align: center;
  border-top: 1px solid var(--line);
  font-family: 'Times New Roman', Times, serif;
  letter-spacing: .01em;
  line-height: 1.4;
}

/* Bold figure numbers within captions */
.figure figcaption strong,
.figure figcaption b {
  color: var(--accent);
  font-weight: 700;
}

.figure-row {
  display: flex;
  justify-content: center;
  gap: 24px;
  flex-wrap: wrap;
  margin: 28px 0;
}

.figure-row .figure {
  margin: 0;
  flex: 1 1 200px;
  max-width: 340px;
}

/* ============================================================
   Index page – Chapter grid
   ============================================================ */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 18px;
  margin-top: 24px;
}

.chapter-card {
  padding: 22px 24px;
  border: 1px solid var(--line);
  background: var(--paper);
  border-radius: var(--radius);
  transition: all .3s var(--ease);
  position: relative;
  overflow: hidden;
  animation: fadeInUp .5s var(--ease) both;
}

/* Stagger animation for cards */
.chapter-card:nth-child(1)  { animation-delay: .05s; }
.chapter-card:nth-child(2)  { animation-delay: .10s; }
.chapter-card:nth-child(3)  { animation-delay: .15s; }
.chapter-card:nth-child(4)  { animation-delay: .20s; }
.chapter-card:nth-child(5)  { animation-delay: .25s; }
.chapter-card:nth-child(6)  { animation-delay: .30s; }
.chapter-card:nth-child(7)  { animation-delay: .35s; }
.chapter-card:nth-child(8)  { animation-delay: .40s; }
.chapter-card:nth-child(9)  { animation-delay: .45s; }
.chapter-card:nth-child(10) { animation-delay: .50s; }
.chapter-card:nth-child(11) { animation-delay: .55s; }
.chapter-card:nth-child(12) { animation-delay: .60s; }

/* Gold left accent stripe on cards */
.chapter-card::before {
  content: "";
  position: absolute;
  left: 0; top: 0; bottom: 0;
  width: 4px;
  background: linear-gradient(180deg, var(--accent), var(--gold));
  transform: scaleY(0);
  transition: transform .3s var(--ease);
  transform-origin: bottom;
}

/* Chapter number badge */
.chapter-card .card-number {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  border-radius: 50%;
  background: var(--accent-light);
  color: var(--accent);
  font-family: 'Times New Roman', Times, serif;
  font-weight: 700;
  font-size: 13pt;
  margin-bottom: 8px;
  transition: all .3s var(--ease);
}

/* Chapter card icon */
.chapter-card .card-icon {
  font-size: 28px;
  margin-bottom: 4px;
  display: block;
  opacity: .65;
  transition: opacity .3s var(--ease), transform .3s var(--ease);
}
.chapter-card:hover .card-icon {
  opacity: 1;
  transform: scale(1.1);
}

.chapter-card:hover {
  box-shadow: var(--shadow-lg);
  background: var(--paper);
  transform: translateY(-4px);
  border-color: var(--gold);
}
[data-theme="dark"] .chapter-card:hover {
  background: var(--paper-edge);
}
.chapter-card:hover::before {
  transform: scaleY(1);
}
.chapter-card:hover .card-number {
  background: var(--accent);
  color: white;
}

.chapter-card h3 {
  border-left: none;
  padding-left: 0;
  margin-top: 0;
  margin-bottom: 4px;
  font-size: var(--fs-h3);
  color: var(--accent);
  text-transform: none;
  letter-spacing: normal;
}

.chapter-card p {
  text-indent: 0;
  margin-bottom: 0;
  color: var(--muted);
  font-size: var(--fs-body);
}

/* Last-visited chapter indicator */
.chapter-card.last-visited {
  border-color: var(--gold);
  box-shadow: 0 0 0 1px var(--gold), var(--shadow-sm);
}
.chapter-card.last-visited::after {
  content: "Continue reading";
  position: absolute;
  top: 8px;
  right: 10px;
  font-family: 'Times New Roman', Times, serif;
  font-size: 8pt;
  text-transform: uppercase;
  letter-spacing: .06em;
  color: var(--gold);
  font-weight: 600;
  opacity: .85;
}

/* ============================================================
   Additional details & micro-interactions
   ============================================================ */

/* --- Ornamental section separator between major sections --- */
h2 ~ h2 {
  margin-top: 52px;
}

::selection {
  background: rgba(26,60,94,.18);
  color: var(--ink);
}

.callout, .formula, .exam-box {
  transition: box-shadow .2s, transform .2s;
}
.callout:hover, .exam-box:hover {
  box-shadow: 0 4px 16px rgba(0,0,0,.08);
}
.formula:hover {
  box-shadow: 0 2px 12px rgba(0,0,0,.06);
}

/* --- Intersection Observer reveal animation --- */
.reveal {
  opacity: 0;
  transform: translateY(20px);
  transition: opacity .6s var(--ease), transform .6s var(--ease);
}
.reveal.visible {
  opacity: 1;
  transform: translateY(0);
}

/* ============================================================
   Chapter Navigation (Prev / Next)
   ============================================================ */
.chapter-nav {
  display: flex;
  justify-content: space-between;
  gap: 16px;
  margin-top: 48px;
  padding-top: 24px;
  border-top: 1px solid var(--line);
}
.chapter-nav a {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 14px 22px;
  border: 1px solid var(--line);
  background: var(--soft);
  border-radius: var(--radius);
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-small);
  color: var(--accent);
  text-decoration: none;
  transition: all var(--dur) var(--ease);
  min-width: 140px;
}
.chapter-nav a:hover {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
  box-shadow: var(--shadow-md);
  transform: translateY(-2px);
}
.chapter-nav .nav-next {
  margin-left: auto;
  text-align: right;
}
.chapter-nav .nav-label {
  display: block;
  font-size: var(--fs-tiny);
  text-transform: uppercase;
  letter-spacing: .1em;
  opacity: .7;
  margin-bottom: 2px;
}
.chapter-nav .nav-title {
  font-weight: 600;
}

/* ============================================================
   Quick-info badges (for chapter pages)
   ============================================================ */
.chapter-meta {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin: 12px 0 4px;
}
.meta-badge {
  display: inline-flex;
  align-items: center;
  gap: 4px;
  padding: 4px 12px;
  border-radius: 20px;
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-tiny);
  font-weight: 600;
  letter-spacing: .03em;
}
.meta-badge.exam {
  background: #87ceeb;
  color: #000000;
  border: 1px solid #5ba8cc;
}
.meta-badge.topic {
  background: var(--accent-light);
  color: var(--accent);
  border: 1px solid rgba(26,60,94,.15);
}

/* ============================================================
   Enhanced exam-ref tags
   ============================================================ */
.exam-ref {
  display: inline-block;
  margin-left: 6px;
  padding: 2px 10px;
  border: none;
  background: #87ceeb;
  color: #000000;
  font-size: var(--fs-tiny);
  font-weight: 600;
  font-family: 'Times New Roman', Times, serif;
  letter-spacing: .02em;
  vertical-align: middle;
  white-space: nowrap;
  border-radius: 12px;
  transition: all .2s var(--ease);
}
.exam-ref:hover {
  background: #5ba8cc;
  color: #000000;
}

/* ============================================================
   Intro / note callout + cross-reference tag (RCC past questions)
   ============================================================ */
.intro-note {
  background: var(--accent-light);
  border-left: 4px solid var(--accent);
  border-radius: 0 var(--radius) var(--radius) 0;
  padding: 14px 20px;
  margin: 20px 0 28px;
}
.intro-note p {
  margin: 0;
  font-size: var(--fs-small);
  line-height: 1.6;
}
.intro-note p + p { margin-top: 10px; }
[data-theme="dark"] .intro-note {
  background: rgba(255,255,255,.04);
  border-left-color: var(--accent);
}
.xref {
  display: inline-block;
  margin-left: 6px;
  padding: 1px 8px;
  border-radius: 10px;
  background: var(--soft);
  border: 1px solid var(--line);
  color: var(--muted);
  font-size: var(--fs-tiny);
  font-style: italic;
  white-space: nowrap;
}
[data-theme="dark"] .xref {
  background: rgba(255,255,255,.06);
  border-color: rgba(255,255,255,.12);
}

/* ============================================================
   Tooltip style for key terms
   ============================================================ */
.key-term {
  border-bottom: 1.5px dashed var(--gold);
  cursor: help;
  position: relative;
}

/* ---- Inline code & preformatted blocks ---- */
code {
  background: var(--soft);
  padding: 1px 5px;
  border-radius: 3px;
  font-family: "Consolas", "Menlo", "Monaco", monospace;
  color: var(--accent-2);
  border: .5px solid var(--line);
}
pre {
  background: var(--soft);
  padding: 16px 18px;
  border-left: 3px solid var(--accent);
  border-radius: 0 var(--radius) var(--radius) 0;
  overflow-x: auto;
  margin: 18px 0;
  line-height: 1.65;
}
pre code {
  background: none;
  border: none;
  padding: 0;
  color: inherit;
}
[data-theme="dark"] code {
  background: var(--paper-edge);
  border-color: var(--line);
}

/* ============================================================
   Statistics / Hero Cards (for index)
   ============================================================ */
.stats-row {
  display: flex;
  justify-content: center;
  gap: 24px;
  flex-wrap: wrap;
  margin: 28px 0;
}
.stat-card {
  text-align: center;
  padding: 16px 24px;
  min-width: 120px;
  border-radius: var(--radius);
  background: var(--soft);
  border: 1px solid var(--line);
  transition: all .3s var(--ease);
}
.stat-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-md);
}
.stat-card .stat-number {
  display: block;
  font-family: 'Times New Roman', Times, serif;
  font-size: 24pt;
  font-weight: 800;
  color: var(--accent);
  line-height: 1.1;
}
.stat-card .stat-label {
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-tiny);
  text-transform: uppercase;
  letter-spacing: .1em;
  color: var(--muted);
  margin-top: 4px;
}

/* ============================================================
   Print styles
   ============================================================ */
@media print {
  body { background: white; }
  .page {
    margin: 0; width: 100%;
    box-shadow: none;
    border: none;
    border-radius: 0;
    animation: none;
    background-image: none;
  }
  .page::before { display: none; }
  .back-link { display: none; }
  .theme-toggle, .scroll-top, .scroll-progress, .chapter-nav { display: none; }
  h2, h3 { page-break-after: avoid; }
  .figure { page-break-inside: avoid; }
  table { page-break-inside: avoid; }
  .callout, .exam-box, .formula { page-break-inside: avoid; box-shadow: none; }
  .chapter-card { break-inside: avoid; }
  a { color: var(--ink); border-bottom: none; }
}

/* ============================================================
   Responsive – small screens
   ============================================================ */
@media (max-width: 760px) {
  :root {
    --gutter: 24px;
    --fs-h1: 20pt;
    --fs-h2: 14pt;
    --fs-h3: 12pt;
  }
  .page {
    width: calc(100vw - 8px);
    min-width: 0;
    margin: 16px auto;
    border-radius: 0;
  }
  .page::before { display: none; }
  .two-col { grid-template-columns: 1fr; }
  nav.toc ol { columns: 1; }
  .chapter-grid { grid-template-columns: 1fr; }
  .stats-row { gap: 12px; }
  .stat-card { min-width: 80px; padding: 12px 16px; }
  .stat-card .stat-number { font-size: 18pt; }
  .theme-toggle { top: 8px; right: 8px; width: 38px; height: 38px; }
  .chapter-nav { flex-direction: column; }
  .drop-cap::first-letter {
    float: none;
    font-size: inherit;
    padding: 0;
  }
  .callout, .formula, .exam-box {
    padding-left: 40px;
  }
  .callout::before, .formula::before, .exam-box::before {
    left: 10px;
  }
  .figure-row {
    flex-direction: column;
    align-items: center;
  }
  .figure-row .figure {
    max-width: 100%;
  }
}

/* ============================================================
   Responsive – wide screens
   ============================================================ */

/* ============================================================
   Mnemonics / Personal Notes
   ============================================================ */
.mnemonic-block {
  margin: 28px 0;
  border-radius: var(--radius);
  position: relative;
}
.mnemonic-block.mnemonic-chapter {
  margin-bottom: 36px;
  padding-bottom: 8px;
  border-bottom: 1px dashed var(--line);
}
.mnemonic-toggle {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  padding: 8px 18px;
  border: 1.5px dashed var(--gold);
  background: var(--soft);
  color: var(--gold);
  cursor: pointer;
  border-radius: 20px;
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-small);
  font-weight: 600;
  letter-spacing: .02em;
  transition: all .25s var(--ease);
}
.mnemonic-toggle:hover {
  background: var(--gold);
  color: white;
  border-style: solid;
}
.mnemonic-toggle.active {
  background: linear-gradient(135deg, var(--gold), #d4a017);
  color: white;
  border-style: solid;
  border-color: var(--gold);
}
.mnemonic-toggle.chapter-level {
  font-size: var(--fs-body);
  padding: 10px 22px;
}
.mnemonic-editor {
  margin-top: 10px;
  padding: 16px 18px;
  background: linear-gradient(135deg, #f8f8f8, #f0f0f0);
  border: 1px solid #f59e0b33;
  border-radius: var(--radius);
  animation: fadeIn .3s var(--ease);
}
[data-theme="dark"] .mnemonic-editor {
  background: linear-gradient(135deg, #22262e, #2a2d33);
  border-color: #f59e0b22;
}
.mnemonic-label {
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-tiny);
  color: var(--gold);
  text-transform: uppercase;
  letter-spacing: .08em;
  margin-bottom: 8px;
  font-weight: 600;
}
.mnemonic-textarea {
  width: 100%;
  min-height: 80px;
  padding: 12px 14px;
  border: 1px solid var(--line);
  border-radius: 6px;
  background: var(--paper);
  color: var(--ink);
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-body);
  line-height: 1.7;
  resize: vertical;
  transition: border-color .2s, box-shadow .2s;
}
.mnemonic-textarea:focus {
  outline: none;
  border-color: var(--gold);
  box-shadow: 0 0 0 3px rgba(184,134,11,.15);
}
.mnemonic-textarea.chapter-textarea {
  min-height: 120px;
}
.mnemonic-saved {
  display: inline-block;
  margin-top: 6px;
  font-family: 'Times New Roman', Times, serif;
  font-size: var(--fs-tiny);
  color: var(--green);
  font-weight: 600;
  transition: opacity .3s;
}
@media print {
  .mnemonic-block { display: none; }
}