:root {
  --primary: #0b3d91;
  --primary-dark: #082a66;
  --accent: #ffb703;
  --bg: #f4f6fb;
  --card: #ffffff;
  --text: #1b2238;
  --muted: #5a6378;
  --border: #e2e6ef;
  --correct: #16a34a;
  --correct-bg: #dcfce7;
  --wrong: #dc2626;
  --wrong-bg: #fee2e2;
  --neutral-hover: #eef2fb;
  --code-bg: #f1f5f9;
  --radius: 10px;
  --shadow: 0 1px 3px rgba(10, 25, 70, 0.06), 0 4px 12px rgba(10, 25, 70, 0.06);
  --font: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: var(--font);
  font-size: 16px;
  line-height: 1.55;
  scroll-behavior: smooth;
}

.layout {
  max-width: 1100px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 1fr;
  gap: 16px;
  padding: 16px;
  padding-bottom: 80px;
}

@media (min-width: 900px) {
  .layout { grid-template-columns: 260px 1fr; gap: 24px; padding: 24px; }
  aside.toc { position: sticky; top: 16px; align-self: start; max-height: calc(100vh - 32px); overflow-y: auto; }
}

header.top {
  grid-column: 1 / -1;
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 14px 18px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 14px;
  flex-wrap: wrap;
  box-shadow: var(--shadow);
}
header.top h1 {
  margin: 0;
  color: var(--primary);
  font-size: 1.2rem;
  letter-spacing: -0.01em;
}
header.top .chapter-meta {
  color: var(--muted);
  font-size: 0.85rem;
}
header.top a.back {
  color: var(--primary);
  text-decoration: none;
  font-size: 0.9rem;
  font-weight: 600;
  padding: 6px 10px;
  border-radius: 6px;
  border: 1px solid var(--border);
  background: #fff;
}
header.top a.back:hover { background: var(--neutral-hover); }

/* TOC */
aside.toc {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 14px;
  box-shadow: var(--shadow);
}
aside.toc h2 { font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.05em; color: var(--muted); margin: 0 0 10px; }
aside.toc ol { list-style: none; padding: 0; margin: 0; counter-reset: section; }
aside.toc li { margin: 4px 0; }
aside.toc a {
  display: block;
  padding: 6px 10px;
  border-radius: 6px;
  text-decoration: none;
  color: var(--text);
  font-size: 0.9rem;
  border-left: 3px solid transparent;
}
aside.toc a:hover { background: var(--neutral-hover); }
aside.toc a.active {
  background: #eff4ff;
  color: var(--primary-dark);
  border-left-color: var(--primary);
  font-weight: 600;
}

/* Sections */
main { min-width: 0; }
section.chapter-section {
  background: var(--card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  margin-bottom: 16px;
  box-shadow: var(--shadow);
  overflow: hidden;
}
section.chapter-section > header {
  padding: 16px 18px;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: linear-gradient(180deg, #f9fafd 0%, #ffffff 100%);
  border-bottom: 1px solid var(--border);
  user-select: none;
}
section.chapter-section.collapsed > header { border-bottom: none; }
section.chapter-section > header h2 {
  margin: 0;
  font-size: 1.05rem;
  color: var(--primary-dark);
  display: flex;
  align-items: center;
  gap: 10px;
}
section.chapter-section > header .sec-num {
  background: var(--primary);
  color: #fff;
  font-size: 0.78rem;
  padding: 3px 9px;
  border-radius: 999px;
  font-weight: 700;
}
section.chapter-section > header .toggle {
  font-size: 1.2rem;
  color: var(--muted);
  transition: transform 0.2s;
}
section.chapter-section.collapsed > header .toggle {
  transform: rotate(-90deg);
}
section.chapter-section > .body {
  padding: 16px 18px 20px;
}
section.chapter-section.collapsed > .body { display: none; }

/* Concept cards */
.concept {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 14px 16px;
  margin-bottom: 14px;
  background: #fcfdff;
}
.concept h3 {
  font-size: 1rem;
  margin: 0 0 8px;
  color: var(--primary-dark);
  display: flex;
  align-items: center;
  gap: 8px;
  flex-wrap: wrap;
}
.concept h3 .sub-ref {
  font-size: 0.7rem;
  color: var(--muted);
  font-weight: 400;
  font-family: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  background: var(--code-bg);
  padding: 2px 6px;
  border-radius: 3px;
}
.concept .explain {
  font-size: 0.93rem;
  line-height: 1.6;
  margin: 0 0 10px;
  color: var(--text);
}
.concept .explain p { margin: 0 0 8px; }
.concept .explain p:last-child { margin-bottom: 0; }
.concept .explain ul, .concept .explain ol { margin: 6px 0 8px; padding-left: 22px; }
.concept .explain li { margin: 2px 0; }
.concept .explain b, .concept .explain strong { color: var(--primary-dark); }
.concept .explain code, code.kbd {
  background: var(--code-bg);
  padding: 1px 5px;
  border-radius: 3px;
  font-size: 0.88em;
  font-family: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
}
.formula {
  background: var(--code-bg);
  padding: 10px 14px;
  border-radius: 6px;
  font-family: ui-monospace, "SF Mono", Menlo, Consolas, monospace;
  font-size: 0.95rem;
  margin: 8px 0;
  text-align: center;
  border-left: 3px solid var(--primary);
}

/* Buttons inside concept */
.concept .actions {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  margin-top: 6px;
}
.concept button {
  font-family: inherit;
  font-size: 0.82rem;
  padding: 6px 10px;
  border-radius: 6px;
  border: 1px solid var(--border);
  background: #fff;
  cursor: pointer;
  color: var(--text);
  transition: background 0.15s;
}
.concept button:hover { background: var(--neutral-hover); }
.concept button.audio-btn.playing {
  background: var(--accent);
  border-color: var(--accent);
  color: var(--primary-dark);
}
.concept button.practise-btn {
  background: var(--primary);
  color: #fff;
  border-color: var(--primary);
}
.concept button.practise-btn:hover { background: var(--primary-dark); }

/* Worked example */
.worked-example {
  border: 1px dashed var(--primary);
  border-radius: 8px;
  padding: 12px 14px;
  margin: 10px 0;
  background: #f5f8ff;
}
.worked-example .we-title {
  font-size: 0.78rem;
  font-weight: 700;
  color: var(--primary);
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 6px;
}
.worked-example .we-question {
  font-weight: 600;
  font-size: 0.93rem;
  margin-bottom: 10px;
  color: var(--text);
}
.worked-example .we-steps {
  padding-left: 0;
  list-style: none;
  margin: 0;
}
.worked-example .we-step {
  margin: 0 0 8px;
  padding: 8px 10px;
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 6px;
  cursor: pointer;
  transition: background 0.15s;
  position: relative;
}
.worked-example .we-step:hover { background: #fafbff; }
.worked-example .we-step.revealed { cursor: default; background: #f9fbff; }
.worked-example .we-step.locked .we-step-body { display: none; }
.worked-example .we-step.locked::before {
  content: "▶ Reveal step";
  color: var(--primary);
  font-size: 0.85rem;
  font-weight: 600;
}
.worked-example .we-step .we-step-num {
  display: inline-block;
  background: var(--primary);
  color: #fff;
  font-size: 0.7rem;
  width: 18px;
  height: 18px;
  line-height: 18px;
  border-radius: 50%;
  text-align: center;
  font-weight: 700;
  margin-right: 6px;
}
.worked-example .we-step .we-step-body {
  font-size: 0.9rem;
  line-height: 1.55;
}
.worked-example .we-step .we-step-body p { margin: 4px 0; }

/* Check question */
.checkq {
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 12px 14px;
  margin-top: 10px;
  background: #fffefa;
  border-left: 4px solid var(--accent);
}
.checkq .cq-label {
  font-size: 0.78rem;
  font-weight: 700;
  color: #92400e;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  margin-bottom: 6px;
}
.checkq .cq-question {
  font-weight: 600;
  font-size: 0.92rem;
  margin-bottom: 8px;
}
.checkq .cq-options {
  display: grid;
  gap: 6px;
}
.checkq .cq-opt {
  text-align: left;
  padding: 8px 10px;
  border: 1px solid var(--border);
  background: #fff;
  border-radius: 6px;
  cursor: pointer;
  font-size: 0.88rem;
  font-family: inherit;
  display: flex;
  gap: 8px;
}
.checkq .cq-opt:hover:not(:disabled) { background: var(--neutral-hover); }
.checkq .cq-opt .letter { font-weight: 700; color: var(--primary); }
.checkq .cq-opt.correct {
  background: var(--correct-bg);
  border-color: var(--correct);
}
.checkq .cq-opt.correct .letter { color: var(--correct); }
.checkq .cq-opt.wrong {
  background: var(--wrong-bg);
  border-color: var(--wrong);
}
.checkq .cq-opt.wrong .letter { color: var(--wrong); }
.checkq .cq-opt:disabled { cursor: default; }
.checkq .cq-feedback {
  margin-top: 8px;
  padding: 8px 10px;
  border-radius: 6px;
  font-size: 0.85rem;
  line-height: 1.5;
}
.checkq .cq-feedback.correct {
  background: var(--correct-bg);
  border-left: 3px solid var(--correct);
}
.checkq .cq-feedback.wrong {
  background: var(--wrong-bg);
  border-left: 3px solid var(--wrong);
}
.checkq .cq-feedback hidden { display: none; }

/* Diagrams */
.diagram {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 8px;
  margin: 8px 0;
  text-align: center;
}
.diagram svg { max-width: 100%; height: auto; }
.diagram .caption {
  font-size: 0.78rem;
  color: var(--muted);
  margin-top: 4px;
}

/* Quick refs */
.quick-ref-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 8px;
  margin: 10px 0;
}
@media (max-width: 600px) {
  .quick-ref-grid { grid-template-columns: 1fr; }
}
.quick-ref-card {
  background: #fff;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 8px 10px;
}
.quick-ref-card .qr-title {
  font-weight: 700;
  font-size: 0.85rem;
  color: var(--primary-dark);
  margin-bottom: 3px;
}
.quick-ref-card .qr-body {
  font-size: 0.83rem;
  color: var(--text);
  line-height: 1.45;
}

/* Misc */
.intro {
  background: #eff4ff;
  border-left: 4px solid var(--primary);
  padding: 12px 14px;
  border-radius: 6px;
  margin-bottom: 14px;
  font-size: 0.92rem;
}
.intro b { color: var(--primary-dark); }
.tip {
  background: #fef3c7;
  border-left: 4px solid #f59e0b;
  padding: 10px 12px;
  border-radius: 6px;
  margin: 10px 0;
  font-size: 0.88rem;
}
.tip::before { content: "💡 "; }
.trap {
  background: #fee2e2;
  border-left: 4px solid var(--wrong);
  padding: 10px 12px;
  border-radius: 6px;
  margin: 10px 0;
  font-size: 0.88rem;
}
.trap::before { content: "⚠️ "; font-weight: 700; }
.trap b { color: var(--wrong); }