:root {
  --bg: #0f1419;
  --panel: #1a2129;
  --panel-2: #232c36;
  --text: #e6edf3;
  --muted: #8b97a3;
  --accent: #4a9eff;
  --accent-hover: #6bb0ff;
  --good: #3fb950;
  --bad: #f85149;
  --warn: #d29922;
  --border: #2d3742;
  --radius: 8px;
}
* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
body {
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
  line-height: 1.5;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
header {
  background: var(--panel);
  border-bottom: 1px solid var(--border);
  padding: 12px 24px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}
header .brand {
  font-weight: 600;
  font-size: 1.1rem;
  color: var(--text);
  text-decoration: none;
}
nav .link-btn {
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  padding: 6px 12px;
  border-radius: var(--radius);
  cursor: pointer;
  font-size: 0.85rem;
}
nav .link-btn:hover { color: var(--text); border-color: var(--accent); }
main {
  flex: 1;
  max-width: 900px;
  width: 100%;
  margin: 0 auto;
  padding: 24px;
}
footer {
  background: var(--panel);
  border-top: 1px solid var(--border);
  color: var(--muted);
  text-align: center;
  padding: 12px;
  font-size: 0.85rem;
}
h1 { margin-top: 0; font-size: 1.6rem; }
h2 { font-size: 1.2rem; margin: 0; }
a { color: var(--accent); text-decoration: none; }
a:hover { color: var(--accent-hover); }
.btn {
  display: inline-block;
  background: var(--accent);
  color: #fff;
  border: none;
  padding: 10px 18px;
  border-radius: var(--radius);
  font-size: 0.95rem;
  cursor: pointer;
  font-weight: 500;
  margin-right: 8px;
  margin-bottom: 8px;
}
.btn:hover:not(:disabled) { background: var(--accent-hover); }
.btn:disabled { opacity: 0.4; cursor: not-allowed; }
.btn.secondary {
  background: var(--panel-2);
  color: var(--text);
  border: 1px solid var(--border);
}
.btn.secondary:hover:not(:disabled) { border-color: var(--accent); }
.btn.danger { background: var(--bad); }
.chapter-list { display: grid; gap: 10px; grid-template-columns: 1fr; }
.filter-row {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 12px 0;
  color: var(--text);
  font-size: 0.9rem;
  cursor: pointer;
  user-select: none;
}
.filter-row input { accent-color: var(--accent); }
.diff-row {
  display: flex;
  align-items: center;
  gap: 12px;
  margin-bottom: 14px;
  padding: 10px 14px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  flex-wrap: wrap;
}
.diff-label { color: var(--muted); font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.05em; }
.diff-group { display: inline-flex; gap: 0; border: 1px solid var(--border); border-radius: var(--radius); overflow: hidden; }
.diff-btn {
  background: var(--panel-2);
  color: var(--text);
  border: none;
  padding: 6px 14px;
  cursor: pointer;
  font-size: 0.9rem;
  border-right: 1px solid var(--border);
}
.diff-btn:last-child { border-right: none; }
.diff-btn:hover { background: #2a3540; }
.diff-btn.active { background: var(--accent); color: #fff; }
.badge {
  display: inline-block;
  background: var(--panel-2);
  color: var(--muted);
  font-size: 0.7rem;
  padding: 1px 6px;
  border-radius: 10px;
  border: 1px solid var(--border);
  margin-left: 4px;
  font-weight: 400;
}
.badge.hard { background: #3a2a1d; color: var(--warn); border-color: #5a3a1d; }
.chapter-card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 14px 18px;
  display: flex;
  justify-content: space-between;
  align-items: center;
  text-decoration: none;
  color: var(--text);
}
.chapter-card:hover { border-color: var(--accent); color: var(--text); }
.chapter-card.final-card {
  border-color: var(--accent);
  background: linear-gradient(135deg, var(--panel) 0%, #1d2a3a 100%);
  margin-bottom: 14px;
}
.chapter-card .ch-title { flex: 1; }
.chapter-card .ch-num { color: var(--muted); font-size: 0.85rem; margin-bottom: 2px; }
.chapter-card .ch-name { font-weight: 500; }
.chapter-card .ch-stats { text-align: right; min-width: 140px; }
.score-bar {
  width: 100px;
  height: 6px;
  background: var(--panel-2);
  border-radius: 3px;
  overflow: hidden;
  display: inline-block;
  vertical-align: middle;
  margin-right: 8px;
}
.score-bar > div { height: 100%; background: var(--good); transition: width 0.3s; }
.score-num { font-variant-numeric: tabular-nums; font-weight: 500; }
.muted { color: var(--muted); font-size: 0.85rem; }
.summary-row {
  display: flex;
  gap: 24px;
  margin-bottom: 16px;
  padding: 14px 18px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  flex-wrap: wrap;
}
.summary-row .stat { min-width: 90px; }
.summary-row .stat-label { color: var(--muted); font-size: 0.8rem; text-transform: uppercase; letter-spacing: 0.05em; }
.summary-row .stat-val { font-size: 1.4rem; font-weight: 600; }
.q-card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 20px 24px;
  margin-bottom: 16px;
}
.q-stem { font-size: 1.05rem; margin-bottom: 16px; font-weight: 500; }
.q-meta { color: var(--muted); font-size: 0.85rem; margin-bottom: 12px; }
.choice {
  display: block;
  background: var(--panel-2);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 12px 16px;
  margin-bottom: 8px;
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s;
  user-select: none;
}
.choice:hover:not(.locked) { border-color: var(--accent); }
.choice.selected { border-color: var(--accent); background: #243340; }
.choice.correct { border-color: var(--good); background: #1c3023; }
.choice.wrong { border-color: var(--bad); background: #3a1d1f; }
.choice.locked { cursor: default; }
.choice .key { color: var(--muted); margin-right: 10px; font-variant-numeric: tabular-nums; }
.feedback {
  margin-top: 14px;
  padding: 12px 14px;
  border-radius: var(--radius);
  border-left: 3px solid var(--accent);
  background: var(--panel-2);
}
.feedback.correct { border-color: var(--good); }
.feedback.wrong { border-color: var(--bad); }
.feedback .label { font-weight: 600; margin-bottom: 4px; }
.feedback .label.correct { color: var(--good); }
.feedback .label.wrong { color: var(--bad); }
.controls { margin-top: 16px; display: flex; align-items: center; gap: 8px; }
.progress { color: var(--muted); margin-left: auto; font-variant-numeric: tabular-nums; }
.q-list { list-style: none; padding: 0; }
.q-list li {
  padding: 8px 12px;
  border-bottom: 1px solid var(--border);
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 12px;
}
.q-list li:last-child { border-bottom: none; }
.q-list .qid { color: var(--muted); font-family: ui-monospace, monospace; font-size: 0.85rem; min-width: 80px; }
.q-list .qstem { flex: 1; font-size: 0.92rem; }
.q-list .qstats { color: var(--muted); font-size: 0.8rem; min-width: 80px; text-align: right; }
.dot { display: inline-block; width: 8px; height: 8px; border-radius: 50%; margin-right: 3px; }
.dot.good { background: var(--good); }
.dot.bad { background: var(--bad); }
.dot.empty { background: var(--panel-2); border: 1px solid var(--border); }
.back { color: var(--muted); font-size: 0.9rem; display: inline-block; margin-bottom: 12px; }
.summary-card {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 24px;
  text-align: center;
}
.summary-card .big-score { font-size: 3rem; font-weight: 700; margin: 12px 0; }
.summary-card .big-score.good { color: var(--good); }
.summary-card .big-score.warn { color: var(--warn); }
.summary-card .big-score.bad { color: var(--bad); }
@media (max-width: 600px) {
  main { padding: 16px; }
  .chapter-card { flex-direction: column; align-items: flex-start; gap: 8px; }
  .chapter-card .ch-stats { text-align: left; }
}