:root {
  --paper: #f5f1e8;
  --paper-2: #ebe5d6;
  --ink: #0f1a2e;
  --ink-soft: #2a3651;
  --ink-mute: #5a6378;
  --rule: #d4cdb8;
  --accent: #b8391f;
  --accent-2: #c5862a;
  --teal: #1e5662;
  --ok: #2f6b3a;
  --shade: #faf6ec;
  --shade-2: #f0ead8;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--paper);
  color: var(--ink);
  font-family: 'Inter Tight', system-ui, sans-serif;
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}

body {
  background-image:
    radial-gradient(circle at 15% 20%, rgba(184, 57, 31, 0.03) 0, transparent 40%),
    radial-gradient(circle at 85% 80%, rgba(30, 86, 98, 0.04) 0, transparent 40%);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.site-header {
  border-bottom: 2px solid var(--ink);
  background: var(--paper);
  position: sticky;
  top: 0;
  z-index: 50;
}
.header-inner {
  max-width: 1200px;
  margin: 0 auto;
  padding: 18px 32px;
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 24px;
}
.brand { display: flex; align-items: center; gap: 14px; cursor: pointer; }
.brand-mark {
  font-family: 'Fraunces', serif;
  font-size: 38px;
  font-weight: 800;
  width: 52px;
  height: 52px;
  display: flex;
  align-items: center;
  justify-content: center;
  background: var(--ink);
  color: var(--paper);
  border-radius: 4px;
  font-style: italic;
}
.brand-title {
  font-family: 'Fraunces', serif;
  font-size: 22px;
  font-weight: 600;
  letter-spacing: -0.01em;
  line-height: 1;
}
.brand-sub {
  font-size: 11px;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  color: var(--ink-mute);
  margin-top: 4px;
}
.top-nav { display: flex; gap: 4px; }
.top-nav a {
  color: var(--ink);
  text-decoration: none;
  padding: 8px 14px;
  font-size: 14px;
  font-weight: 500;
  letter-spacing: 0.02em;
  border-radius: 3px;
  transition: all 0.15s;
  cursor: pointer;
}
.top-nav a:hover { background: var(--ink); color: var(--paper); }
.top-nav a.active { background: var(--accent); color: var(--paper); }

main {
  max-width: 1200px;
  margin: 0 auto;
  padding: 48px 32px 80px;
  width: 100%;
  flex: 1;
}

.hero {
  display: grid;
  grid-template-columns: 1.4fr 1fr;
  gap: 48px;
  align-items: start;
  padding: 24px 0 56px;
  border-bottom: 1px solid var(--rule);
  margin-bottom: 56px;
}
.hero-label {
  font-size: 11px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 600;
  margin-bottom: 20px;
}
.hero h1 {
  font-family: 'Fraunces', serif;
  font-size: clamp(44px, 6vw, 76px);
  line-height: 0.98;
  letter-spacing: -0.03em;
  margin: 0 0 24px;
  font-weight: 600;
}
.hero h1 em {
  font-style: italic;
  color: var(--accent);
  font-weight: 400;
}
.hero-lede {
  font-size: 18px;
  color: var(--ink-soft);
  max-width: 520px;
  margin-bottom: 32px;
}
.hero-cta {
  display: inline-block;
  background: var(--ink);
  color: var(--paper);
  padding: 14px 28px;
  text-decoration: none;
  font-weight: 500;
  letter-spacing: 0.02em;
  font-size: 15px;
  border-radius: 2px;
  transition: all 0.15s;
  cursor: pointer;
}
.hero-cta:hover { background: var(--accent); transform: translateY(-1px); }

.hero-aside {
  border: 1px solid var(--ink);
  padding: 28px;
  background: var(--shade);
  position: relative;
}
.hero-aside::before {
  content: '';
  position: absolute;
  top: -1px; left: -1px;
  width: 24px; height: 24px;
  border-top: 3px solid var(--accent);
  border-left: 3px solid var(--accent);
}
.aside-label {
  font-size: 10px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--ink-mute);
  margin-bottom: 12px;
}
.aside-eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 13px;
  color: var(--ink);
  line-height: 2;
}
.aside-eq div { padding: 6px 0; border-bottom: 1px dashed var(--rule); }
.aside-eq div:last-child { border-bottom: none; }

.section-head {
  display: flex;
  align-items: baseline;
  justify-content: space-between;
  margin-bottom: 28px;
  padding-bottom: 12px;
  border-bottom: 2px solid var(--ink);
}
.section-head h2 {
  font-family: 'Fraunces', serif;
  font-size: 36px;
  font-weight: 600;
  letter-spacing: -0.02em;
  margin: 0;
}
.section-head .pill {
  font-size: 11px;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--ink-mute);
}

.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(320px, 1fr));
  gap: 20px;
}
.chapter-card {
  background: var(--shade);
  border: 1px solid var(--rule);
  padding: 28px;
  cursor: pointer;
  transition: all 0.2s;
  position: relative;
  overflow: hidden;
  text-decoration: none;
  color: inherit;
  display: block;
}
.chapter-card:hover {
  background: var(--ink);
  color: var(--paper);
  transform: translateY(-2px);
  box-shadow: 6px 6px 0 var(--accent);
}
.chapter-card:hover .ch-num { color: var(--accent-2); }
.chapter-card:hover .ch-topics { color: var(--paper); opacity: 0.8; }
.chapter-card:hover .ch-title { color: var(--paper); }
.ch-num {
  font-family: 'Fraunces', serif;
  font-style: italic;
  font-size: 14px;
  color: var(--accent);
  margin-bottom: 8px;
  letter-spacing: 0.04em;
}
.ch-title {
  font-family: 'Fraunces', serif;
  font-size: 26px;
  font-weight: 600;
  letter-spacing: -0.01em;
  line-height: 1.1;
  margin-bottom: 16px;
  color: var(--ink);
}
.ch-topics {
  font-size: 13px;
  color: var(--ink-mute);
  line-height: 1.5;
}

.chapter-header {
  padding: 32px 0 40px;
  border-bottom: 2px solid var(--ink);
  margin-bottom: 48px;
}
.chapter-kicker {
  font-size: 11px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 600;
  margin-bottom: 12px;
}
.chapter-title {
  font-family: 'Fraunces', serif;
  font-size: clamp(40px, 5vw, 64px);
  font-weight: 600;
  letter-spacing: -0.025em;
  line-height: 1;
  margin: 0 0 16px;
}
.chapter-sub {
  font-size: 17px;
  color: var(--ink-soft);
  max-width: 680px;
}

.chapter-body {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 56px;
  align-items: start;
}
.toc {
  position: sticky;
  top: 100px;
  font-size: 13px;
}
.toc-label {
  font-size: 10px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--ink-mute);
  margin-bottom: 12px;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--rule);
}
.toc a {
  display: block;
  color: var(--ink-soft);
  text-decoration: none;
  padding: 6px 0;
  border-bottom: 1px dashed var(--rule);
  transition: color 0.15s;
  cursor: pointer;
}
.toc a:hover { color: var(--accent); }

.content-col { min-width: 0; }
.content-col h3 {
  font-family: 'Fraunces', serif;
  font-size: 28px;
  font-weight: 600;
  letter-spacing: -0.015em;
  margin: 48px 0 16px;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--rule);
  scroll-margin-top: 100px;
}
.content-col h3:first-child { margin-top: 0; }
.content-col h4 {
  font-family: 'Fraunces', serif;
  font-size: 19px;
  font-weight: 600;
  margin: 28px 0 10px;
  color: var(--ink-soft);
}
.content-col p {
  font-size: 16px;
  margin: 0 0 14px;
  color: var(--ink);
}
.content-col ul, .content-col ol {
  padding-left: 22px;
  margin: 10px 0 16px;
}
.content-col li { margin-bottom: 6px; }
.content-col code {
  font-family: 'JetBrains Mono', monospace;
  background: var(--shade-2);
  padding: 1px 6px;
  font-size: 0.92em;
  border-radius: 2px;
}
.content-col strong { color: var(--accent); font-weight: 600; }

.worked {
  background: var(--shade-2);
  border: 1px solid var(--rule);
  padding: 24px 28px;
  margin: 28px 0;
  position: relative;
}
.worked-label {
  font-size: 10px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--accent);
  font-weight: 700;
  margin-bottom: 8px;
}
.worked-title {
  font-family: 'Fraunces', serif;
  font-size: 19px;
  font-weight: 600;
  margin-bottom: 12px;
}
.worked p { font-size: 15px; }
.worked .soln {
  margin-top: 12px;
  padding-top: 12px;
  border-top: 1px dashed var(--rule);
}
.soln-label {
  font-weight: 600;
  font-size: 12px;
  color: var(--teal);
  letter-spacing: 0.08em;
  margin-bottom: 6px;
  text-transform: uppercase;
}

.callout {
  border: 1px solid var(--ink);
  background: var(--paper);
  padding: 18px 22px;
  margin: 20px 0;
  position: relative;
  font-size: 15px;
}
.callout::before {
  content: 'KEY IDEA';
  position: absolute;
  top: -9px;
  left: 14px;
  background: var(--accent);
  color: var(--paper);
  font-size: 10px;
  letter-spacing: 0.2em;
  padding: 2px 8px;
  font-weight: 700;
}

.practice-section {
  margin-top: 64px;
  padding-top: 40px;
  border-top: 3px double var(--ink);
}
.practice-section h3 {
  font-family: 'Fraunces', serif;
  font-size: 32px;
  margin-bottom: 6px;
}
.practice-intro {
  color: var(--ink-mute);
  margin-bottom: 32px;
  font-size: 15px;
}

.problem {
  background: var(--shade);
  border: 1px solid var(--rule);
  padding: 22px 26px;
  margin-bottom: 18px;
  border-left: 3px solid var(--ink);
}
.problem.conceptual { border-left-color: var(--teal); }
.problem.mcq { border-left-color: var(--accent-2); }

.prob-header {
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  margin-bottom: 10px;
  gap: 12px;
}
.prob-num {
  font-family: 'Fraunces', serif;
  font-style: italic;
  font-size: 18px;
  color: var(--accent);
  font-weight: 600;
}
.prob-type {
  font-size: 10px;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--ink-mute);
  font-weight: 600;
}
.prob-body { font-size: 15px; margin-bottom: 10px; }
.prob-choices {
  list-style: none;
  padding: 0;
  margin: 12px 0;
}
.prob-choices li {
  padding: 4px 0;
  font-size: 15px;
  padding-left: 28px;
  position: relative;
}
.prob-choices li::before {
  content: attr(data-letter) ')';
  font-weight: 600;
  color: var(--accent);
  position: absolute;
  left: 0;
}

.toggle-ans {
  background: none;
  border: 1px solid var(--ink);
  color: var(--ink);
  padding: 6px 14px;
  font-family: inherit;
  font-size: 12px;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  font-weight: 500;
  cursor: pointer;
  margin-top: 8px;
  transition: all 0.15s;
}
.toggle-ans:hover { background: var(--ink); color: var(--paper); }
.answer {
  display: none;
  margin-top: 12px;
  padding: 14px 16px;
  background: var(--paper);
  border-left: 2px solid var(--ok);
  font-size: 14px;
}
.answer.open { display: block; }
.answer-label {
  font-weight: 700;
  color: var(--ok);
  font-size: 11px;
  letter-spacing: 0.15em;
  text-transform: uppercase;
  margin-bottom: 6px;
}

.chapter-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 64px;
  padding-top: 32px;
  border-top: 1px solid var(--rule);
  gap: 16px;
}
.nav-btn {
  flex: 1;
  padding: 20px;
  border: 1px solid var(--ink);
  text-decoration: none;
  color: var(--ink);
  transition: all 0.15s;
  cursor: pointer;
}
.nav-btn:hover { background: var(--ink); color: var(--paper); }
.nav-btn.next { text-align: right; }
.nav-btn .nav-dir {
  font-size: 10px;
  letter-spacing: 0.25em;
  text-transform: uppercase;
  color: var(--ink-mute);
  display: block;
  margin-bottom: 4px;
}
.nav-btn:hover .nav-dir { color: var(--paper); opacity: 0.7; }
.nav-btn .nav-title {
  font-family: 'Fraunces', serif;
  font-size: 18px;
  font-weight: 600;
}

.frq-intro-card {
  background: var(--ink);
  color: var(--paper);
  padding: 32px 36px;
  margin-bottom: 40px;
  position: relative;
}
.frq-intro-card h2 {
  font-family: 'Fraunces', serif;
  font-size: 36px;
  margin: 0 0 8px;
  font-weight: 600;
  letter-spacing: -0.02em;
}
.frq-intro-card p {
  opacity: 0.85;
  font-size: 15px;
  max-width: 640px;
  margin: 0;
}

.frq-item {
  background: var(--shade);
  border: 1px solid var(--rule);
  padding: 32px 36px;
  margin-bottom: 28px;
  border-top: 4px solid var(--accent);
}
.frq-item h3 {
  font-family: 'Fraunces', serif;
  font-size: 22px;
  margin: 0 0 4px;
}
.frq-meta {
  font-size: 11px;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--ink-mute);
  margin-bottom: 18px;
}
.frq-stem {
  font-size: 15px;
  margin-bottom: 18px;
  padding-bottom: 18px;
  border-bottom: 1px dashed var(--rule);
}
.frq-part {
  margin-bottom: 16px;
  padding-left: 18px;
  border-left: 2px solid var(--rule);
}
.frq-part-label {
  font-weight: 600;
  color: var(--accent);
  font-family: 'Fraunces', serif;
  font-style: italic;
}
.rubric {
  margin-top: 14px;
  padding: 14px 18px;
  background: var(--paper);
  border: 1px dashed var(--ink-soft);
  font-size: 14px;
}
.rubric-label {
  font-weight: 700;
  font-size: 11px;
  letter-spacing: 0.2em;
  text-transform: uppercase;
  color: var(--teal);
  margin-bottom: 6px;
}
.rubric ul { margin: 4px 0 0; padding-left: 18px; }

.formula-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(290px, 1fr));
  gap: 18px;
}
.formula-card {
  background: var(--shade);
  border: 1px solid var(--rule);
  padding: 20px 22px;
  border-top: 3px solid var(--ink);
}
.formula-card h4 {
  font-family: 'Fraunces', serif;
  font-size: 18px;
  margin: 0 0 12px;
  color: var(--accent);
}
.formula-card ul {
  list-style: none;
  padding: 0;
  margin: 0;
}
.formula-card li {
  padding: 10px 0;
  border-bottom: 1px dashed var(--rule);
}
.formula-card li:last-child { border-bottom: none; }
.formula-card li .name {
  font-size: 12px;
  color: var(--ink-mute);
  letter-spacing: 0.02em;
  display: block;
  margin-bottom: 4px;
}
.formula-card li .eq {
  font-family: 'JetBrains Mono', monospace;
  font-size: 14px;
  color: var(--ink);
}

.site-footer {
  border-top: 2px solid var(--ink);
  background: var(--shade-2);
  padding: 24px 0;
  margin-top: auto;
}
.footer-inner {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 32px;
  display: flex;
  justify-content: space-between;
  font-size: 12px;
  color: var(--ink-mute);
  letter-spacing: 0.02em;
}

@media (max-width: 900px) {
  .hero { grid-template-columns: 1fr; gap: 32px; }
  .chapter-body { grid-template-columns: 1fr; }
  .toc { position: static; margin-bottom: 24px; padding: 16px; background: var(--shade); border: 1px solid var(--rule); }
  .header-inner { flex-direction: column; gap: 14px; align-items: flex-start; }
  main { padding: 32px 20px 60px; }
  .footer-inner { flex-direction: column; gap: 8px; }
}

/* Expanded content additions */
.mistake-box {
  background: #fdf0ec;
  border: 1px solid var(--accent);
  border-left: 4px solid var(--accent);
  padding: 18px 22px;
  margin: 24px 0;
  font-size: 15px;
}
.mistake-box::before {
  content: 'COMMON MISTAKE';
  display: block;
  font-size: 10px;
  letter-spacing: 0.25em;
  color: var(--accent);
  font-weight: 700;
  margin-bottom: 8px;
}
.mistake-box strong { color: var(--ink); }

.strategy-box {
  background: #ecf3f5;
  border: 1px solid var(--teal);
  border-left: 4px solid var(--teal);
  padding: 20px 24px;
  margin: 24px 0;
  font-size: 15px;
}
.strategy-box::before {
  content: 'EXAM STRATEGY';
  display: block;
  font-size: 10px;
  letter-spacing: 0.25em;
  color: var(--teal);
  font-weight: 700;
  margin-bottom: 10px;
}
.strategy-box strong { color: var(--ink); }
.strategy-box ol, .strategy-box ul { margin: 6px 0 0; padding-left: 22px; }

.depth-note {
  background: var(--paper);
  border: 1px dashed var(--ink-mute);
  padding: 14px 18px;
  margin: 18px 0;
  font-size: 14px;
  font-style: italic;
  color: var(--ink-soft);
}
.depth-note strong { font-style: normal; color: var(--ink); }

.problem.hard { border-left-color: var(--accent); }
.problem.hard .prob-type::after {
  content: ' · HARDER';
  color: var(--accent);
  font-weight: 700;
}

.prob-section-label {
  font-family: 'Fraunces', serif;
  font-size: 18px;
  font-weight: 600;
  color: var(--ink-soft);
  margin: 32px 0 14px;
  padding-bottom: 6px;
  border-bottom: 1px dashed var(--rule);
  letter-spacing: -0.01em;
}
.prob-section-label:first-of-type { margin-top: 20px; }