:root {
  --bg: #f4f6fb;
  --card-bg: #ffffff;
  --primary: #2b6cb0;
  --primary-hover: #1f4e84;
  --success: #2f855a;
  --danger: #c53030;
  --text: #1a202c;
  --muted: #718096;
  --border: #e2e8f0;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.5;
}

.container {
  max-width: 720px;
  margin: 32px auto;
  padding: 0 16px;
}

h1 { margin: 0 0 16px; }
h2 { margin: 0 0 12px; }

.lead {
  color: var(--muted);
  margin-bottom: 24px;
}

.muted { color: var(--muted); }
.hint { color: var(--muted); margin-top: 8px; font-size: 0.9rem; }

.card {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 20px;
  margin-bottom: 16px;
  box-shadow: 0 1px 2px rgba(0,0,0,0.03);
}

.btn-row {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}

.btn {
  display: inline-block;
  padding: 10px 16px;
  border-radius: 6px;
  border: 1px solid var(--border);
  background: #fff;
  color: var(--text);
  font-size: 0.95rem;
  cursor: pointer;
  text-decoration: none;
  transition: background 0.15s ease;
}

.btn:hover { background: #f7fafc; }
.btn:disabled { opacity: 0.5; cursor: not-allowed; }

.btn-primary {
  background: var(--primary);
  color: #fff;
  border-color: var(--primary);
}
.btn-primary:hover { background: var(--primary-hover); }

.btn-small {
  padding: 6px 12px;
  font-size: 0.85rem;
}

.top-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 16px;
  padding: 12px 16px;
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 8px;
}

.options {
  display: flex;
  flex-direction: column;
  gap: 8px;
  margin: 16px 0;
}

.option-btn {
  text-align: left;
  padding: 12px 14px;
  border: 1px solid var(--border);
  border-radius: 6px;
  background: #fff;
  cursor: pointer;
  font-size: 1rem;
}

.option-btn:hover:not(:disabled) { background: #f7fafc; }
.option-btn:disabled { cursor: default; }

.option-btn.correct {
  background: #f0fff4;
  border-color: var(--success);
  color: var(--success);
  font-weight: 600;
}

.option-btn.wrong {
  background: #fff5f5;
  border-color: var(--danger);
  color: var(--danger);
  font-weight: 600;
}

.result {
  margin-top: 16px;
  padding-top: 16px;
  border-top: 1px solid var(--border);
}

.result-line {
  font-size: 1.1rem;
  font-weight: 600;
  margin: 0 0 8px;
}

.result-line.correct { color: var(--success); }
.result-line.wrong { color: var(--danger); }

.ref-box {
  background: #f7fafc;
  border-left: 3px solid var(--primary);
  padding: 10px 14px;
  margin: 12px 0;
  border-radius: 4px;
}

.ref-box p { margin: 4px 0 0; }

.progress-bar {
  width: 100%;
  height: 8px;
  background: var(--border);
  border-radius: 4px;
  overflow: hidden;
  margin-top: 4px;
}

#progress-fill {
  height: 100%;
  background: var(--primary);
  width: 0%;
  transition: width 0.3s ease;
}

#progress-text {
  font-size: 0.85rem;
  color: var(--muted);
}

.stats-table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 12px;
}

.stats-table th, .stats-table td {
  padding: 8px 10px;
  text-align: left;
  border-bottom: 1px solid var(--border);
  font-size: 0.95rem;
}

.stats-table th {
  background: #f7fafc;
  font-weight: 600;
}

.chapter-list {
  display: flex;
  flex-direction: column;
  gap: 6px;
  margin-top: 8px;
  max-height: 280px;
  overflow-y: auto;
}

.chapter-list label,
.chapter-all {
  display: flex;
  align-items: center;
  gap: 8px;
  padding: 6px 8px;
  border-radius: 4px;
  cursor: pointer;
}

.chapter-list label:hover,
.chapter-all:hover {
  background: #f7fafc;
}

.chapter-list label {
  border: 1px solid var(--border);
}

.chapter-list input:disabled + span {
  color: var(--muted);
}

.chapter-all {
  border-bottom: 1px solid var(--border);
  padding-bottom: 10px;
  margin-bottom: 4px;
  font-weight: 600;
}