:root {
  --bg: #0f1117;
  --bg-elev: #171a23;
  --bg-card: #1d2130;
  --border: #2a2f42;
  --text: #e6e8ef;
  --text-dim: #9ba0b4;
  --accent: #ff6b4a;
  --accent-soft: #ff8b6a;
  --good: #37d399;
  --bad: #ff5c7a;
  --warn: #ffcf5c;
  --shadow: 0 8px 32px rgba(0, 0, 0, 0.35);
  --radius: 14px;
  --radius-sm: 8px;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Noto Sans", Arial, sans-serif;
  line-height: 1.6;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

.container {
  max-width: 960px;
  margin: 0 auto;
  padding: 24px 20px 80px;
}

/* ── Top bar ─────────────────────────────────────────── */
.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 20px;
  background: var(--bg-elev);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
  backdrop-filter: blur(8px);
}

.topbar .brand {
  font-weight: 700;
  font-size: 18px;
  color: var(--text);
  text-decoration: none;
  letter-spacing: 0.3px;
}

.topbar .brand span { color: var(--accent); }

.topbar .back-link {
  color: var(--text-dim);
  text-decoration: none;
  font-size: 14px;
  padding: 6px 12px;
  border-radius: var(--radius-sm);
  transition: background 0.15s, color 0.15s;
  display: inline-flex;
  align-items: center;
  gap: 5px;
  line-height: 1;
}

.btn-arrow {
  display: inline-block;
  line-height: 1;
  font-size: 1em;
  /* nudges the arrow glyph onto the same optical baseline as Latin text */
  transform: translateY(-1px);
}

.topbar .back-link:hover {
  background: var(--bg-card);
  color: var(--text);
}

.topbar .crumb {
  font-size: 13px;
  color: var(--text-dim);
}

/* ── Hero ────────────────────────────────────────────── */
.hero {
  padding: 40px 0 28px;
  text-align: center;
}

.hero h1 {
  font-size: 38px;
  margin: 0 0 12px;
  letter-spacing: -0.5px;
}

.hero h1 .emoji { font-size: 42px; }

.hero p {
  color: var(--text-dim);
  font-size: 17px;
  max-width: 620px;
  margin: 0 auto;
}

/* ── Key panel ───────────────────────────────────────── */
.key-panel {
  margin: 24px 0;
  padding: 20px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

.key-panel h3 {
  margin: 0 0 6px;
  font-size: 16px;
}

.key-panel p {
  margin: 0 0 14px;
  font-size: 14px;
  color: var(--text-dim);
}

.key-panel .row {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
}

.key-panel input {
  flex: 1;
  min-width: 240px;
  background: var(--bg);
  border: 1px solid var(--border);
  color: var(--text);
  padding: 10px 14px;
  border-radius: var(--radius-sm);
  font-family: "SF Mono", "Menlo", monospace;
  font-size: 13px;
}

.key-panel input:focus {
  outline: none;
  border-color: var(--accent);
}

.key-panel .key-status {
  margin-top: 10px;
  font-size: 13px;
  color: var(--text-dim);
}

.key-panel .key-status.ok { color: var(--good); }
.key-panel .key-status.warn { color: var(--warn); }

/* ── Buttons ─────────────────────────────────────────── */
.btn {
  background: var(--accent);
  color: #fff;
  border: none;
  padding: 10px 18px;
  border-radius: var(--radius-sm);
  font-weight: 600;
  font-size: 14px;
  cursor: pointer;
  transition: background 0.15s, transform 0.05s;
  font-family: inherit;
  text-decoration: none;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 6px;
  line-height: 1.2;
  vertical-align: middle;
}

.btn:hover { text-decoration: none; }

.btn:hover { background: var(--accent-soft); }
.btn:active { transform: translateY(1px); }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }

.btn.ghost {
  background: transparent;
  color: var(--text);
  border: 1px solid var(--border);
}

.btn.ghost:hover {
  background: var(--bg-card);
  border-color: var(--accent);
}

/* ── Chapter grid (home) ─────────────────────────────── */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(270px, 1fr));
  gap: 16px;
  margin-top: 28px;
}

.chapter-card {
  display: block;
  text-decoration: none;
  color: inherit;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px;
  transition: transform 0.12s, border-color 0.12s, box-shadow 0.12s;
}

.chapter-card:hover {
  transform: translateY(-2px);
  border-color: var(--accent);
  box-shadow: 0 10px 24px rgba(255, 107, 74, 0.08);
}

.chapter-card .num {
  display: inline-block;
  font-size: 12px;
  font-weight: 700;
  color: var(--accent);
  letter-spacing: 1px;
  margin-bottom: 8px;
}

.chapter-card h3 {
  margin: 0 0 10px;
  font-size: 19px;
}

.chapter-card p {
  margin: 0;
  font-size: 14px;
  color: var(--text-dim);
}

.chapter-card .icon {
  font-size: 30px;
  margin-bottom: 8px;
  display: block;
}

/* ── Chapter page ────────────────────────────────────── */
.chapter-header {
  padding: 30px 0 20px;
}

.chapter-header .label {
  font-size: 13px;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: 1px;
  margin-bottom: 6px;
}

.chapter-header h1 {
  font-size: 32px;
  margin: 0 0 8px;
  letter-spacing: -0.4px;
}

.chapter-header p {
  color: var(--text-dim);
  font-size: 16px;
  margin: 0;
  max-width: 720px;
}

/* ── Section (syllabus sub-point) ────────────────────── */
.section {
  margin: 32px 0;
  padding: 22px 24px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
}

.section h2 {
  margin: 0 0 14px;
  font-size: 20px;
  color: var(--text);
}

.section h2 .num {
  color: var(--accent);
  font-family: "SF Mono", "Menlo", monospace;
  font-size: 16px;
  margin-right: 10px;
}

.section h3 {
  margin: 18px 0 8px;
  font-size: 16px;
  color: var(--text);
}

.section p { margin: 8px 0; }

.section .definition {
  color: var(--text);
}

.section .hook {
  background: rgba(255, 207, 92, 0.08);
  border-left: 3px solid var(--warn);
  padding: 12px 16px;
  margin: 14px 0;
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: 15px;
}

.section .hook strong { color: var(--warn); }

.section ul {
  padding-left: 22px;
  margin: 10px 0;
}

.section ul li { margin: 6px 0; }

.section table {
  width: 100%;
  border-collapse: collapse;
  margin: 12px 0;
  font-size: 14px;
}

.section table th, .section table td {
  border: 1px solid var(--border);
  padding: 8px 12px;
  text-align: left;
}

.section table th {
  background: var(--bg-elev);
  font-weight: 600;
}

/* ── Quiz panel ──────────────────────────────────────── */
.quiz-panel {
  margin: 40px 0 10px;
  padding: 28px;
  background: linear-gradient(135deg, rgba(255, 107, 74, 0.07), rgba(255, 107, 74, 0.02));
  border: 1px solid var(--border);
  border-radius: var(--radius);
  text-align: center;
}

.quiz-panel h2 {
  margin: 0 0 8px;
  font-size: 22px;
}

.quiz-panel p {
  color: var(--text-dim);
  margin: 0 0 18px;
  font-size: 14px;
}

.quiz-panel .btn + .btn { margin-left: 10px; }

/* ── Quiz questions ──────────────────────────────────── */
.quiz-status {
  font-size: 14px;
  color: var(--text-dim);
  margin-bottom: 14px;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.quiz-status .score { font-weight: 600; color: var(--text); }

.quiz-question {
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px 22px;
  margin-bottom: 14px;
  text-align: left;
}

.quiz-question .q-num {
  display: inline-block;
  font-size: 12px;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: 1px;
  margin-bottom: 8px;
}

.quiz-question .q-text {
  font-weight: 600;
  margin: 0 0 14px;
  font-size: 16px;
  line-height: 1.5;
}

.quiz-option {
  display: block;
  width: 100%;
  text-align: left;
  padding: 11px 16px;
  margin: 7px 0;
  background: var(--bg);
  border: 1px solid var(--border);
  color: var(--text);
  border-radius: var(--radius-sm);
  font-size: 14px;
  cursor: pointer;
  transition: border-color 0.12s, background 0.12s;
  font-family: inherit;
  line-height: 1.45;
}

.quiz-option:hover:not(:disabled) {
  border-color: var(--accent);
  background: var(--bg-elev);
}

.quiz-option:disabled { cursor: not-allowed; }

.quiz-option.correct {
  border-color: var(--good);
  background: rgba(55, 211, 153, 0.12);
  color: var(--text);
}

.quiz-option.wrong {
  border-color: var(--bad);
  background: rgba(255, 92, 122, 0.12);
  color: var(--text);
}

.quiz-option .marker {
  display: inline-block;
  width: 22px;
  font-weight: 700;
  color: var(--text-dim);
}

.quiz-option.correct .marker { color: var(--good); }
.quiz-option.wrong .marker { color: var(--bad); }

.quiz-explanation {
  margin-top: 10px;
  padding: 10px 14px;
  background: var(--bg);
  border-left: 3px solid var(--accent);
  font-size: 13.5px;
  color: var(--text-dim);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
}

.quiz-explanation strong { color: var(--text); }

/* ── Quiz footer (result) ────────────────────────────── */
.quiz-result {
  margin-top: 24px;
  padding: 22px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  text-align: center;
}

.quiz-result h3 {
  margin: 0 0 10px;
  font-size: 22px;
}

.quiz-result .big-score {
  font-size: 42px;
  font-weight: 700;
  color: var(--accent);
  margin: 8px 0;
}

.quiz-result p { color: var(--text-dim); margin: 6px 0 16px; }

.quiz-result .btn + .btn { margin-left: 10px; }

/* ── Loading + error ─────────────────────────────────── */
.quiz-loading {
  padding: 40px;
  text-align: center;
  color: var(--text-dim);
  font-size: 14px;
}

.quiz-loading .spinner {
  display: inline-block;
  width: 22px;
  height: 22px;
  border: 3px solid var(--border);
  border-top-color: var(--accent);
  border-radius: 50%;
  animation: spin 0.8s linear infinite;
  margin-right: 10px;
  vertical-align: middle;
}

@keyframes spin { to { transform: rotate(360deg); } }

.quiz-error {
  padding: 18px 22px;
  background: rgba(255, 92, 122, 0.08);
  border: 1px solid var(--bad);
  border-radius: var(--radius);
  color: var(--bad);
  font-size: 14px;
  margin: 20px 0;
}

.quiz-error strong { color: var(--bad); }

/* ── Utility ─────────────────────────────────────────── */
.muted { color: var(--text-dim); }
.tiny { font-size: 12px; }
.center { text-align: center; }
.mt-20 { margin-top: 20px; }
.mb-0 { margin-bottom: 0; }

/* ── Subjective question cards (5/10 mark) ──────────── */
.subj-question {
  padding: 22px 24px;
}

.subj-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 10px;
}

.subj-header .q-num {
  margin: 0;
}

.marks-badge {
  display: inline-block;
  padding: 4px 12px;
  border-radius: 999px;
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 1px;
  text-transform: uppercase;
}

.marks-badge.five {
  background: rgba(55, 211, 153, 0.15);
  color: var(--good);
  border: 1px solid rgba(55, 211, 153, 0.3);
}

.marks-badge.ten {
  background: rgba(255, 107, 74, 0.15);
  color: var(--accent);
  border: 1px solid rgba(255, 107, 74, 0.35);
}

.subj-reveal-btn {
  font-size: 13px !important;
  padding: 7px 14px !important;
}

.subj-guide {
  margin-top: 16px;
  padding: 16px 18px;
  background: var(--bg);
  border: 1px dashed var(--border);
  border-radius: var(--radius-sm);
}

.subj-guide h4 {
  margin: 14px 0 8px;
  font-size: 14px;
  color: var(--accent);
  letter-spacing: 0.3px;
}

.subj-guide h4:first-child { margin-top: 0; }

.subj-guide .approach-list,
.subj-guide .keypoints-list {
  padding-left: 22px;
  margin: 4px 0;
}

.subj-guide .approach-list li,
.subj-guide .keypoints-list li {
  margin: 5px 0;
  font-size: 14px;
  color: var(--text);
}

.subj-guide .approach-list strong {
  color: var(--accent);
  margin-right: 2px;
}

/* ── Paper section header on home ────────────────────── */
.paper-header {
  margin: 40px 0 6px;
  display: flex;
  align-items: baseline;
  gap: 12px;
  flex-wrap: wrap;
}

.paper-header h2 {
  margin: 0;
  font-size: 22px;
}

.paper-header .tag {
  font-size: 11px;
  font-weight: 700;
  letter-spacing: 1.2px;
  padding: 3px 10px;
  border-radius: 999px;
  background: var(--bg-card);
  border: 1px solid var(--border);
  color: var(--text-dim);
  text-transform: uppercase;
}

.paper-header .tag.mcq { color: var(--good); border-color: rgba(55, 211, 153, 0.35); }
.paper-header .tag.subj { color: var(--accent); border-color: rgba(255, 107, 74, 0.4); }

/* ── Verify link (official source) ────────────────────── */
.verify-link {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  margin-top: 10px;
  padding: 6px 12px;
  background: rgba(55, 211, 153, 0.1);
  border: 1px solid rgba(55, 211, 153, 0.35);
  border-radius: var(--radius-sm);
  color: var(--good);
  text-decoration: none;
  font-size: 13px;
  font-weight: 500;
  transition: background 0.15s;
}

.verify-link:hover {
  background: rgba(55, 211, 153, 0.2);
  color: var(--good);
}

.verify-link::before {
  content: "🔗";
  font-size: 12px;
}

.verify-note {
  margin: 16px 0;
  padding: 12px 16px;
  background: rgba(255, 207, 92, 0.07);
  border: 1px solid rgba(255, 207, 92, 0.3);
  border-radius: var(--radius-sm);
  font-size: 13px;
  color: var(--text);
}

.verify-note strong { color: var(--warn); }

/* ── Responsive: tablets ──────────────────────────────── */
@media (max-width: 900px) {
  .container { padding: 20px 16px 60px; }

  /* Tables: make horizontally scrollable */
  .section table {
    display: block;
    max-width: 100%;
    overflow-x: auto;
    -webkit-overflow-scrolling: touch;
  }
}

/* ── Responsive: phones ───────────────────────────────── */
@media (max-width: 600px) {
  html, body { font-size: 15px; }

  .container { padding: 14px 12px 50px; }

  /* Topbar: tighter spacing, smaller text; keep brand centered */
  .topbar {
    padding: 10px 12px;
    gap: 8px;
  }
  .topbar .brand { font-size: 15px; }
  .topbar .back-link {
    font-size: 13px;
    padding: 5px 8px;
  }
  .topbar .crumb {
    font-size: 11px;
    text-align: right;
  }

  /* Hero */
  .hero { padding: 24px 0 18px; }
  .hero h1 { font-size: 26px; line-height: 1.25; }
  .hero h1 .emoji { font-size: 30px; }
  .hero p { font-size: 14.5px; }

  /* Key panel */
  .key-panel { padding: 16px; }
  .key-panel h3 { font-size: 15px; }
  .key-panel p { font-size: 13px; }
  .key-panel input {
    min-width: 0;
    width: 100%;
    flex: 1 1 100%;
  }
  .key-panel .row .btn,
  .key-panel .row .btn.ghost {
    flex: 1 1 auto;
  }

  /* Chapter grid: single column */
  .chapter-grid {
    grid-template-columns: 1fr;
    gap: 12px;
  }
  .chapter-card { padding: 18px; }
  .chapter-card h3 { font-size: 17px; }
  .chapter-card p { font-size: 13.5px; }
  .chapter-card .icon { font-size: 26px; }

  /* Paper section header */
  .paper-header h2 { font-size: 18px; }
  .paper-header .tag { font-size: 10px; padding: 2px 8px; }

  /* Chapter header */
  .chapter-header { padding: 22px 0 14px; }
  .chapter-header h1 { font-size: 23px; line-height: 1.25; }
  .chapter-header p { font-size: 14.5px; }

  /* Sections */
  .section { padding: 16px 16px; margin: 22px 0; }
  .section h2 { font-size: 18px; }
  .section h2 .num { font-size: 14px; margin-right: 6px; }
  .section h3 { font-size: 15px; margin: 14px 0 6px; }
  .section .hook { padding: 10px 14px; font-size: 14px; }

  /* Quiz panel: stack buttons */
  .quiz-panel { padding: 20px 16px; }
  .quiz-panel h2 { font-size: 19px; }
  .quiz-panel .btn,
  .quiz-panel .btn.ghost {
    display: block;
    width: 100%;
    margin: 8px 0 0 !important;
  }

  /* Quiz status row: allow wrap */
  .quiz-status {
    flex-direction: column;
    align-items: flex-start;
    gap: 4px;
    font-size: 13px;
  }

  /* Quiz questions */
  .quiz-question { padding: 16px 16px; }
  .quiz-question .q-text { font-size: 15px; }
  .quiz-option {
    padding: 10px 12px;
    font-size: 13.5px;
  }
  .quiz-explanation { font-size: 13px; padding: 10px 12px; }

  /* Quiz result: stack buttons */
  .quiz-result { padding: 18px 16px; }
  .quiz-result .big-score { font-size: 34px; }
  .quiz-result .btn,
  .quiz-result .btn.ghost {
    display: block;
    width: 100%;
    margin: 8px 0 0 !important;
  }

  /* Subjective question cards */
  .subj-question { padding: 16px; }
  .subj-header {
    flex-direction: row;
    flex-wrap: wrap;
    gap: 8px;
  }
  .subj-reveal-btn {
    width: 100%;
    margin-top: 8px;
  }
  .subj-guide { padding: 14px; }
  .subj-guide h4 { font-size: 13px; }
  .subj-guide .approach-list,
  .subj-guide .keypoints-list { padding-left: 20px; }
  .subj-guide .approach-list li,
  .subj-guide .keypoints-list li { font-size: 13.5px; }

  /* Generic button row below quiz: stack */
  #quiz .center .btn,
  #practice .center .btn {
    display: block;
    width: 100%;
    margin: 8px 0 0 !important;
  }
}

/* ── Responsive: very small phones (iPhone SE, etc.) ─── */
@media (max-width: 380px) {
  .topbar .crumb { display: none; }
  .hero h1 { font-size: 22px; }
  .chapter-header h1 { font-size: 20px; }
  .section { padding: 14px 14px; }
  .quiz-question { padding: 14px; }
  .quiz-option .marker { width: 18px; }
}