:root {
  --bg: #f5f6f8;
  --card-bg: #ffffff;
  --text: #1a1d23;
  --text-muted: #6b7280;
  --border: #e2e5ea;
  --accent: #6b46c1;
  --accent-dark: #553c9a;
  --green: #16a34a;
  --green-bg: #dcfce7;
  --red: #dc2626;
  --red-bg: #fee2e2;
  --yellow-bg: #fef9c3;
  --radius: 10px;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #14161a;
    --card-bg: #1e2128;
    --text: #e8e9ec;
    --text-muted: #9ca3af;
    --border: #2d3138;
    --accent: #9f7aea;
    --accent-dark: #b794f4;
    --green: #4ade80;
    --green-bg: #14351f;
    --red: #f87171;
    --red-bg: #3a1a1a;
    --yellow-bg: #3a3418;
  }
}

:root[data-theme="dark"] {
  --bg: #14161a;
  --card-bg: #1e2128;
  --text: #e8e9ec;
  --text-muted: #9ca3af;
  --border: #2d3138;
  --accent: #9f7aea;
  --accent-dark: #b794f4;
  --green: #4ade80;
  --green-bg: #14351f;
  --red: #f87171;
  --red-bg: #3a1a1a;
  --yellow-bg: #3a3418;
}

:root[data-theme="light"] {
  --bg: #f5f6f8;
  --card-bg: #ffffff;
  --text: #1a1d23;
  --text-muted: #6b7280;
  --border: #e2e5ea;
  --accent: #6b46c1;
  --accent-dark: #553c9a;
  --green: #16a34a;
  --green-bg: #dcfce7;
  --red: #dc2626;
  --red-bg: #fee2e2;
  --yellow-bg: #fef9c3;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  -webkit-tap-highlight-color: transparent;
}

#app {
  max-width: 760px;
  margin: 0 auto;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

.topbar {
  position: sticky;
  top: 0;
  z-index: 10;
  background: var(--card-bg);
  border-bottom: 1px solid var(--border);
  padding: 12px 16px 0;
}

.topbar h1 {
  font-size: 1.1rem;
  margin: 0 0 10px;
  font-weight: 700;
}

.category-toggle {
  display: flex;
  gap: 6px;
  margin-bottom: 10px;
}

.category-btn {
  flex: 1;
  padding: 8px 0;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 999px;
  font-size: 0.85rem;
  font-weight: 600;
  color: var(--text-muted);
  cursor: pointer;
}

.category-btn.active {
  background: var(--accent);
  border-color: var(--accent);
  color: #fff;
}

.tabs {
  display: flex;
  gap: 4px;
}

.tab-btn {
  flex: 1;
  padding: 10px 0;
  background: none;
  border: none;
  border-bottom: 3px solid transparent;
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text-muted);
  cursor: pointer;
}

.tab-btn.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
}

.tab-panel {
  display: none;
  flex: 1;
  flex-direction: column;
  padding: 12px 16px 32px;
}

.tab-panel.active {
  display: flex;
}

/* Study tab */
.study-controls {
  display: flex;
  gap: 8px;
  margin-bottom: 12px;
  flex-wrap: wrap;
  align-items: center;
}

#search-input {
  flex: 1;
  min-width: 140px;
  padding: 10px 12px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  background: var(--card-bg);
  color: var(--text);
  font-size: 0.95rem;
}

#exam-filter, #quiz-exam-filter {
  padding: 10px 8px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  background: var(--card-bg);
  color: var(--text);
  font-size: 0.9rem;
}

.count-badge {
  font-size: 0.85rem;
  color: var(--text-muted);
  white-space: nowrap;
}

.review-toggle {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 0.85rem;
  color: var(--text-muted);
  cursor: pointer;
  white-space: nowrap;
  user-select: none;
}

.review-toggle input {
  cursor: pointer;
}

.mark-btn {
  background: none;
  border: none;
  cursor: pointer;
  font-size: 1.25rem;
  line-height: 1;
  padding: 2px 4px;
  color: var(--border);
}

.mark-btn.marked {
  color: #f5b301;
}

.q-meta .mark-btn {
  margin-left: auto;
}

.study-list {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.q-card {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 14px 16px;
}

.q-meta {
  display: flex;
  gap: 8px;
  align-items: center;
  margin-bottom: 8px;
  flex-wrap: wrap;
}

.badge {
  font-size: 0.72rem;
  font-weight: 700;
  padding: 2px 8px;
  border-radius: 999px;
  background: var(--accent);
  color: white;
  text-transform: uppercase;
  letter-spacing: 0.03em;
}

.badge-num {
  font-size: 0.8rem;
  color: var(--text-muted);
  font-weight: 600;
}

.badge-flag {
  font-size: 0.7rem;
  padding: 2px 8px;
  border-radius: 999px;
  background: var(--yellow-bg);
  color: var(--text);
}

.q-stem {
  font-size: 0.98rem;
  line-height: 1.5;
  margin: 0 0 8px;
  white-space: pre-wrap;
}

.q-bullets {
  margin: 0 0 8px;
  padding-left: 20px;
  font-size: 0.92rem;
  color: var(--text-muted);
}

.q-code {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 10px 12px;
  font-family: "SF Mono", Monaco, Consolas, monospace;
  font-size: 0.82rem;
  overflow-x: auto;
  white-space: pre;
  margin: 0 0 8px;
}

.q-choose-hint {
  font-size: 0.8rem;
  color: var(--text-muted);
  font-style: italic;
  margin-bottom: 6px;
}

.q-options {
  display: flex;
  flex-direction: column;
  gap: 6px;
  margin-bottom: 10px;
}

.q-option {
  display: flex;
  gap: 8px;
  padding: 8px 10px;
  border-radius: 8px;
  border: 1px solid var(--border);
  font-size: 0.9rem;
  line-height: 1.4;
  align-items: flex-start;
}

.q-option.correct-answer {
  background: var(--green-bg);
  border-color: var(--green);
  color: var(--text);
}

.q-option .opt-letter {
  font-weight: 700;
  min-width: 18px;
}

.q-option .opt-check {
  margin-left: auto;
  color: var(--green);
  font-weight: 700;
}

.why-toggle {
  margin-bottom: 8px;
}

.q-explanation {
  font-size: 0.86rem;
  color: var(--text-muted);
  background: var(--bg);
  border-radius: 8px;
  padding: 10px 12px;
  line-height: 1.5;
  margin-bottom: 8px;
}

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

.q-explanation-detail {
  border: 1px dashed var(--border);
  font-size: 0.82rem;
}

#detail-toggle, .detail-toggle {
  margin-bottom: 8px;
}

/* Quiz tab */
.quiz-controls {
  display: flex;
  gap: 8px;
  align-items: center;
  margin-bottom: 10px;
  flex-wrap: wrap;
}

.btn-secondary, .btn-primary {
  padding: 9px 14px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  background: var(--card-bg);
  color: var(--text);
  font-size: 0.88rem;
  font-weight: 600;
  cursor: pointer;
}

.btn-primary {
  background: var(--accent);
  border-color: var(--accent);
  color: white;
}

.btn-primary:disabled, .btn-secondary:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

.quiz-score {
  margin-left: auto;
  font-size: 0.9rem;
  font-weight: 700;
  color: var(--accent);
}

.quiz-progress {
  margin-bottom: 14px;
}

.progress-bar {
  height: 6px;
  background: var(--border);
  border-radius: 999px;
  overflow: hidden;
  margin-bottom: 4px;
}

.progress-fill {
  height: 100%;
  background: var(--accent);
  width: 0%;
  transition: width 0.25s ease;
}

.progress-text {
  font-size: 0.78rem;
  color: var(--text-muted);
}

.quiz-card {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 18px 16px;
  flex: 1;
  min-height: 200px;
}

.quiz-option {
  display: flex;
  gap: 10px;
  align-items: flex-start;
  padding: 12px 14px;
  border-radius: 10px;
  border: 1.5px solid var(--border);
  margin-bottom: 8px;
  cursor: pointer;
  font-size: 0.95rem;
  line-height: 1.45;
  background: var(--card-bg);
  transition: border-color 0.15s, background 0.15s;
}

.quiz-option:active {
  transform: scale(0.99);
}

.quiz-option.selected {
  border-color: var(--accent);
  background: color-mix(in srgb, var(--accent) 12%, var(--card-bg));
}

.quiz-option.correct {
  border-color: var(--green);
  background: var(--green-bg);
}

.quiz-option.incorrect {
  border-color: var(--red);
  background: var(--red-bg);
}

.quiz-option.disabled {
  cursor: default;
}

.quiz-option .opt-marker {
  font-weight: 700;
  min-width: 20px;
}

.feedback-banner {
  margin: 12px 0;
  padding: 10px 14px;
  border-radius: 8px;
  font-weight: 700;
  font-size: 0.92rem;
}

.feedback-banner.correct {
  background: var(--green-bg);
  color: var(--green);
}

.feedback-banner.incorrect {
  background: var(--red-bg);
  color: var(--red);
}

.quiz-nav {
  display: flex;
  gap: 8px;
  margin-top: 12px;
}

.quiz-nav button {
  flex: 1;
}

.empty-state {
  text-align: center;
  color: var(--text-muted);
  padding: 40px 20px;
}