/* ── Formula Booklet Styles ───────────────────────────── */

.fb-wrap {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  padding-bottom: 3rem;
}

/* Header */
.fb-header {
  background: var(--bg-primary, #fff);
  border-bottom: 1px solid var(--border-color, #e5e7eb);
  padding: 1.5rem 2rem 1rem;
  display: flex;
  align-items: center;
  gap: 2rem;
  flex-wrap: wrap;
  position: sticky;
  top: 0;
  z-index: 50;
}

.fb-header-inner {
  flex: 1;
  min-width: 0;
}

.fb-title {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--text-primary, #111);
  margin: 0 0 0.15rem;
  line-height: 1.2;
}

.fb-subtitle {
  font-size: 0.85rem;
  color: var(--text-secondary, #6b7280);
  margin: 0;
}

.fb-search-wrap {
  flex: 0 0 320px;
  max-width: 100%;
}

.fb-search {
  width: 100%;
  padding: 0.55rem 1rem;
  border: 1.5px solid var(--border-color, #d1d5db);
  border-radius: 999px;
  font-size: 0.9rem;
  background: var(--bg-secondary, #f9fafb);
  color: var(--text-primary, #111);
  outline: none;
  transition: border-color 0.2s, box-shadow 0.2s;
}

.fb-search:focus {
  border-color: var(--color-primary, #6366f1);
  box-shadow: 0 0 0 3px rgba(99,102,241,0.12);
}

/* Layout */
.fb-layout {
  display: grid;
  grid-template-columns: 220px 1fr;
  gap: 0;
  flex: 1;
  min-height: 0;
}

/* Sidebar */
.fb-sidebar {
  background: var(--bg-secondary, #f9fafb);
  border-right: 1px solid var(--border-color, #e5e7eb);
  position: sticky;
  top: 73px;
  height: calc(100vh - 73px);
  overflow-y: auto;
  overflow-x: hidden;
}

.fb-sidebar-inner {
  padding: 1rem 0;
}

.fb-topic-btn {
  display: flex;
  align-items: center;
  gap: 0.6rem;
  width: 100%;
  padding: 0.6rem 1.2rem;
  background: none;
  border: none;
  text-align: left;
  cursor: pointer;
  color: var(--text-primary, #374151);
  font-size: 0.85rem;
  font-weight: 500;
  border-left: 3px solid transparent;
  transition: background 0.15s, border-color 0.15s, color 0.15s;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.fb-topic-btn:hover {
  background: var(--bg-hover, #f3f4f6);
}

.fb-topic-btn.active {
  border-left-color: var(--color-primary, #6366f1);
  background: rgba(99,102,241,0.07);
  color: var(--color-primary, #6366f1);
  font-weight: 600;
}

.fb-topic-icon {
  font-size: 1rem;
  flex-shrink: 0;
}

.fb-topic-count {
  margin-left: auto;
  font-size: 0.7rem;
  background: var(--bg-hover, #e5e7eb);
  border-radius: 999px;
  padding: 0.1rem 0.45rem;
  color: var(--text-secondary, #6b7280);
  flex-shrink: 0;
}

.fb-topic-btn.active .fb-topic-count {
  background: rgba(99,102,241,0.15);
  color: var(--color-primary, #6366f1);
}

.fb-derivations-btn {
  border-left-color: transparent;
}

.fb-derivations-btn.active {
  border-left-color: #6366f1;
  background: rgba(99,102,241,0.07);
  color: #6366f1;
}

.fb-sidebar-divider {
  height: 1px;
  background: var(--border-color, #e5e7eb);
  margin: 0.4rem 1rem;
}

.fb-card-topic-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.25rem;
  font-size: 0.65rem;
  font-weight: 600;
  opacity: 0.75;
  margin-top: 0.1rem;
}

/* Main area */
.fb-main {
  padding: 1.5rem 2rem;
  overflow-y: auto;
  min-height: 0;
}

/* Loading */
.fb-loading {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 200px;
  color: var(--text-secondary, #6b7280);
  gap: 1rem;
}

.fb-spinner {
  width: 36px;
  height: 36px;
  border: 3px solid var(--border-color, #e5e7eb);
  border-top-color: var(--color-primary, #6366f1);
  border-radius: 50%;
  animation: fb-spin 0.8s linear infinite;
}

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

/* Card derives badge */
.fb-card-derives-badge {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  font-size: 0.68rem;
  font-weight: 600;
  color: var(--color-primary, #6366f1);
  background: rgba(99,102,241,0.08);
  border-radius: 4px;
  padding: 0.15rem 0.45rem;
  margin-top: 0.4rem;
  letter-spacing: 0.02em;
}

/* Premium gate */
.fb-premium-gate {
  background: linear-gradient(135deg, rgba(99,102,241,0.06), rgba(139,92,246,0.06));
  border: 1.5px dashed rgba(99,102,241,0.3);
  border-radius: 10px;
  padding: 1.5rem;
  text-align: center;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.5rem;
}

.fb-premium-lock-icon {
  font-size: 1.2rem;
  color: var(--color-primary, #6366f1);
  opacity: 0.7;
}

.fb-premium-gate-title {
  font-size: 0.9rem;
  font-weight: 700;
  color: var(--text-primary, #111);
  margin: 0;
}

.fb-premium-gate-body {
  font-size: 0.82rem;
  color: var(--text-secondary, #6b7280);
  margin: 0;
}

.fb-premium-gate-btn {
  display: inline-block;
  margin-top: 0.25rem;
  padding: 0.45rem 1.1rem;
  background: var(--color-primary, #6366f1);
  color: #fff;
  border-radius: 6px;
  font-size: 0.82rem;
  font-weight: 600;
  text-decoration: none;
  transition: background 0.15s;
}

.fb-premium-gate-btn:hover { background: #4f46e5; color: #fff; }

.fb-premium-gate-btn--small {
  padding: 0.35rem 0.85rem;
  font-size: 0.78rem;
  margin-top: 0;
}

/* Locked exercise */
.fb-exercise-locked {
  position: relative;
  overflow: hidden;
}

.fb-exercise-lock-overlay {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  flex-wrap: wrap;
  margin-top: 0.75rem;
  padding: 0.75rem 1rem;
  background: rgba(99,102,241,0.06);
  border: 1.5px dashed rgba(99,102,241,0.25);
  border-radius: 8px;
  font-size: 0.83rem;
  color: var(--text-secondary, #6b7280);
}

.fb-exercise-lock-overlay i {
  color: var(--color-primary, #6366f1);
  flex-shrink: 0;
}

.fb-exercise-lock-overlay span { flex: 1; }

/* Grid header */
.fb-grid-header {
  margin-bottom: 1.25rem;
}

.fb-grid-title {
  font-size: 1.2rem;
  font-weight: 700;
  color: var(--text-primary, #111);
  margin: 0 0 0.2rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.fb-grid-subtitle {
  font-size: 0.82rem;
  color: var(--text-secondary, #6b7280);
  margin: 0;
}

/* Formula grid */
.fb-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(170px, 220px));
  gap: 0.65rem;
}

/* Formula card */
.fb-card {
  background: var(--bg-primary, #fff);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 10px;
  padding: 0.85rem 0.9rem 0.75rem;
  cursor: pointer;
  transition: border-color 0.2s, transform 0.15s, box-shadow 0.2s;
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  position: relative;
  overflow: hidden;
}

.fb-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: var(--card-accent, #6366f1);
  transform: scaleX(0);
  transform-origin: left;
  transition: transform 0.2s;
}

.fb-card:hover {
  border-color: var(--color-primary, #6366f1);
  transform: translateY(-2px);
  box-shadow: 0 4px 16px rgba(0,0,0,0.08);
}

.fb-card:hover::before {
  transform: scaleX(1);
}

.fb-card-formula {
  font-size: 0.92rem;
  font-weight: 600;
  color: var(--text-primary, #111);
  line-height: 1.3;
  min-height: 2.2rem;
  display: flex;
  align-items: center;
}

.fb-card-formula .katex {
  font-size: 0.95rem;
}

.fb-card-name {
  font-size: 0.7rem;
  color: var(--text-secondary, #6b7280);
  font-weight: 500;
  line-height: 1.3;
}

.fb-card-vars {
  display: flex;
  flex-wrap: wrap;
  gap: 0.25rem;
  margin-top: 0.2rem;
}

.fb-card-var {
  display: inline-block;
  padding: 0.1rem 0.35rem;
  border-radius: 4px;
  font-size: 0.7rem;
  font-weight: 600;
  color: #fff;
  line-height: 1.4;
}

/* ── Formula Detail View ─────────────────────────────── */
.fb-detail {
  width: 100%;
}

.fb-back-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  color: var(--color-primary, #6366f1);
  background: none;
  border: none;
  cursor: pointer;
  font-size: 0.82rem;
  font-weight: 500;
  padding: 0.35rem 0;
  margin-bottom: 0.85rem;
  transition: opacity 0.15s;
}

.fb-back-btn:hover { opacity: 0.7; }

.fb-detail-header {
  margin-bottom: 1rem;
}

.fb-detail-name {
  font-size: 1.4rem;
  font-weight: 700;
  color: var(--text-primary, #111);
  margin: 0 0 0.2rem;
  line-height: 1.2;
}

.fb-detail-topic {
  font-size: 0.75rem;
  color: var(--text-secondary, #6b7280);
  margin: 0;
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  display: flex;
  align-items: center;
  gap: 0.35rem;
}

/* Two-column detail grid */
.fb-detail-grid {
  display: grid;
  grid-template-columns: 1fr 340px;
  gap: 1.5rem;
  align-items: start;
}

.fb-detail-grid--full {
  grid-template-columns: 1fr;
  max-width: 720px;
}

.fb-detail-left {
  min-width: 0;
}

.fb-detail-right {
  min-width: 0;
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

/* Big formula display */
.fb-formula-hero {
  background: var(--bg-secondary, #f9fafb);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 12px;
  padding: 1rem 1.5rem;
  text-align: center;
  margin-bottom: 1rem;
  font-size: 1.4rem;
}

.fb-formula-hero .katex-display {
  margin: 0;
  font-size: 1.8rem;
}

/* Variable chips */
.fb-vars-row {
  display: flex;
  flex-wrap: wrap;
  gap: 0.4rem;
  margin-bottom: 0.85rem;
  align-items: center;
}

.fb-vars-label {
  font-size: 0.78rem;
  color: var(--text-secondary, #6b7280);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-right: 0.25rem;
}

.fb-var-chip {
  display: inline-flex;
  align-items: center;
  gap: 0.35rem;
  padding: 0.3rem 0.8rem;
  border-radius: 999px;
  border: 2px solid transparent;
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  transition: transform 0.15s, box-shadow 0.15s, border-color 0.15s;
  color: #fff;
  user-select: none;
}

.fb-var-chip:hover {
  transform: translateY(-1px);
  box-shadow: 0 2px 8px rgba(0,0,0,0.15);
}

.fb-var-chip.active {
  border-color: #fff;
  box-shadow: 0 0 0 3px rgba(0,0,0,0.15);
}

.fb-var-chip .chip-symbol {
  font-size: 0.95rem;
}

.fb-var-chip .chip-name {
  font-weight: 400;
  font-size: 0.75rem;
  opacity: 0.9;
}

/* Variable detail panel */
.fb-var-panel {
  background: var(--bg-primary, #fff);
  border: 2px solid;
  border-radius: 10px;
  padding: 0.75rem 1rem;
  margin-bottom: 0.85rem;
  display: none;
  gap: 0.6rem;
}

.fb-var-panel.visible {
  display: grid;
  grid-template-columns: auto 1fr;
  align-items: start;
}

.fb-var-panel-symbol {
  font-size: 2.5rem;
  font-weight: 700;
  line-height: 1;
  margin-right: 0.25rem;
  min-width: 3rem;
  text-align: center;
}

.fb-var-panel-info {}

.fb-var-panel-name {
  font-size: 1rem;
  font-weight: 700;
  margin: 0 0 0.15rem;
  color: var(--text-primary, #111);
}

.fb-var-panel-unit {
  font-size: 0.8rem;
  color: var(--text-secondary, #6b7280);
  margin: 0 0 0.4rem;
  font-family: monospace;
}

.fb-var-panel-desc {
  font-size: 0.88rem;
  color: var(--text-secondary, #374151);
  margin: 0;
  line-height: 1.5;
}

/* Two-column: animation + conditions */
.fb-two-col {
  display: grid;
  grid-template-columns: 240px 1fr;
  gap: 0.85rem;
  margin-bottom: 1rem;
  align-items: start;
}

/* Animation panel */
.fb-anim-panel {
  background: #0f172a;
  border-radius: 12px;
  overflow: hidden;
  position: relative;
  aspect-ratio: 4/3;
}

.fb-anim-canvas {
  width: 100%;
  height: 100%;
  display: block;
}

.fb-anim-label {
  position: absolute;
  bottom: 0.5rem;
  left: 0.5rem;
  font-size: 0.65rem;
  color: rgba(255,255,255,0.5);
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

/* Conditions panel */
.fb-conditions {
  background: var(--bg-secondary, #f9fafb);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 12px;
  padding: 1rem 1.2rem;
}

.fb-section-title {
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-secondary, #6b7280);
  margin: 0 0 0.6rem;
}

.fb-conditions-list {
  list-style: none;
  padding: 0;
  margin: 0;
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

.fb-conditions-list li {
  display: flex;
  align-items: flex-start;
  gap: 0.5rem;
  font-size: 0.85rem;
  color: var(--text-primary, #374151);
  line-height: 1.4;
}

.fb-conditions-list li::before {
  content: '';
  display: inline-block;
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: #22c55e;
  flex-shrink: 0;
  margin-top: 0.45em;
}

/* Content sections */
.fb-sections {
  display: flex;
  flex-direction: column;
  gap: 0.65rem;
  margin-bottom: 0;
}

.fb-section {
  background: var(--bg-primary, #fff);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 10px;
  padding: 0.85rem 1rem;
}

.fb-section-body {
  font-size: 0.9rem;
  color: var(--text-primary, #374151);
  line-height: 1.65;
  margin: 0;
}

.fb-section-body .katex {
  font-size: 0.95rem;
}

/* Related formulas */
.fb-related {
  display: flex;
  flex-wrap: wrap;
  gap: 0.5rem;
}

.fb-related-btn {
  display: inline-flex;
  align-items: center;
  gap: 0.3rem;
  padding: 0.35rem 0.85rem;
  border: 1.5px solid var(--color-primary, #6366f1);
  border-radius: 999px;
  background: none;
  color: var(--color-primary, #6366f1);
  font-size: 0.8rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, color 0.15s;
}

.fb-related-btn:hover {
  background: var(--color-primary, #6366f1);
  color: #fff;
}

/* ── Exercise Section ────────────────────────────────── */
.fb-exercises-section {
  background: var(--bg-secondary, #f9fafb);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 10px;
  padding: 1rem 1.1rem;
}

.fb-exercises-title {
  font-size: 1rem;
  font-weight: 700;
  color: var(--text-primary, #111);
  margin: 0 0 0.25rem;
  letter-spacing: 0.03em;
  text-transform: uppercase;
  font-size: 0.78rem;
  color: var(--text-secondary, #6b7280);
}

.fb-exercises-intro {
  font-size: 0.88rem;
  color: var(--text-secondary, #6b7280);
  margin: 0 0 1rem;
}

.fb-exercise-card {
  background: linear-gradient(135deg, rgba(99,102,241,0.04), rgba(139,92,246,0.04));
  border: 1.5px solid rgba(99,102,241,0.2);
  border-radius: 14px;
  padding: 1.25rem 1.5rem;
  margin-bottom: 1rem;
}

.fb-exercise-prompt {
  font-size: 0.95rem;
  font-weight: 600;
  color: var(--text-primary, #111);
  margin: 0 0 0.5rem;
  line-height: 1.5;
}

.fb-exercise-prompt .katex {
  font-size: 1rem;
}

.fb-exercise-context {
  font-size: 0.82rem;
  color: var(--text-secondary, #6b7280);
  margin: 0 0 1rem;
  line-height: 1.5;
}

.fb-exercise-input-row {
  display: flex;
  gap: 0.6rem;
  align-items: stretch;
  flex-wrap: wrap;
}

.fb-exercise-input {
  flex: 1;
  min-width: 180px;
  padding: 0.55rem 0.9rem;
  border: 1.5px solid var(--border-color, #d1d5db);
  border-radius: 8px;
  font-size: 0.9rem;
  background: var(--bg-primary, #fff);
  color: var(--text-primary, #111);
  outline: none;
  transition: border-color 0.2s;
  font-family: 'Courier New', monospace;
}

.fb-exercise-input:focus {
  border-color: var(--color-primary, #6366f1);
}

.fb-exercise-input.correct {
  border-color: #22c55e;
  background: rgba(34,197,94,0.05);
}

.fb-exercise-input.wrong {
  border-color: #ef4444;
  background: rgba(239,68,68,0.05);
}

.fb-exercise-btns {
  display: flex;
  gap: 0.5rem;
  flex-wrap: wrap;
  margin-top: 0.6rem;
}

.fb-check-btn {
  padding: 0.5rem 1.2rem;
  background: var(--color-primary, #6366f1);
  color: #fff;
  border: none;
  border-radius: 8px;
  font-size: 0.85rem;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.15s, transform 0.1s;
}

.fb-check-btn:hover { background: #4f46e5; }
.fb-check-btn:active { transform: scale(0.97); }

.fb-hint-btn,
.fb-reveal-btn {
  padding: 0.5rem 1rem;
  background: none;
  border: 1.5px solid var(--border-color, #d1d5db);
  border-radius: 8px;
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  color: var(--text-secondary, #6b7280);
  transition: border-color 0.15s, color 0.15s;
}

.fb-hint-btn:hover,
.fb-reveal-btn:hover {
  border-color: var(--color-primary, #6366f1);
  color: var(--color-primary, #6366f1);
}

.fb-exercise-feedback {
  margin-top: 0.75rem;
  padding: 0.75rem 1rem;
  border-radius: 8px;
  font-size: 0.88rem;
  line-height: 1.5;
  display: none;
}

.fb-exercise-feedback.visible { display: block; }

.fb-exercise-feedback.fb-correct {
  background: rgba(34,197,94,0.1);
  border: 1px solid rgba(34,197,94,0.3);
  color: #15803d;
}

.fb-exercise-feedback.fb-wrong {
  background: rgba(239,68,68,0.08);
  border: 1px solid rgba(239,68,68,0.25);
  color: #b91c1c;
}

.fb-exercise-feedback.fb-hint {
  background: rgba(245,158,11,0.08);
  border: 1px solid rgba(245,158,11,0.25);
  color: #92400e;
}

.fb-exercise-feedback.fb-reveal {
  background: rgba(99,102,241,0.07);
  border: 1px solid rgba(99,102,241,0.2);
  color: var(--text-primary, #374151);
}

.fb-feedback-answer {
  margin-top: 0.4rem;
  font-size: 0.95rem;
}

/* ── Derives Section ─────────────────────────────────── */
.fb-derives-section {
  background: var(--bg-secondary, #f9fafb);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 10px;
  padding: 1rem 1.1rem;
  margin-bottom: 0;
}

.fb-derives-intro {
  font-size: 0.83rem;
  color: var(--text-secondary, #6b7280);
  margin: 0.2rem 0 1rem;
}

.fb-derives-grid {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.fb-derive-card {
  background: var(--bg-primary, #fff);
  border: 1.5px solid var(--border-color, #e5e7eb);
  border-radius: 10px;
  padding: 1rem 1.25rem;
}

.fb-derive-combo {
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 0.5rem 0.75rem;
  margin-bottom: 0.6rem;
}

.fb-derive-f {
  background: var(--bg-secondary, #f3f4f6);
  border-radius: 6px;
  padding: 0.2rem 0.6rem;
  font-size: 0.9rem;
}

.fb-derive-f .katex {
  font-size: 0.95rem;
}

.fb-derive-op {
  font-size: 0.75rem;
  color: var(--text-secondary, #6b7280);
  font-weight: 500;
  text-transform: uppercase;
  letter-spacing: 0.04em;
}

.fb-derive-arrow {
  font-size: 1.1rem;
  color: var(--color-primary, #6366f1);
  font-weight: 700;
}

.fb-derive-result {
  background: rgba(99,102,241,0.07);
  border: 1.5px solid rgba(99,102,241,0.2);
  border-radius: 6px;
  padding: 0.2rem 0.75rem;
  font-weight: 600;
}

.fb-derive-result .katex-display {
  margin: 0;
  font-size: 1rem;
}

.fb-derive-method {
  font-size: 0.82rem;
  color: var(--text-secondary, #6b7280);
  margin: 0 0 0.3rem;
  line-height: 1.5;
  font-style: italic;
}

.fb-derive-meaning {
  font-size: 0.87rem;
  color: var(--text-primary, #374151);
  margin: 0;
  line-height: 1.5;
}

/* Grid icon */
.fb-grid-icon {
  font-size: 1rem;
  margin-right: 0.25rem;
}

/* Improve formula hero for complex formulas */
.fb-formula-hero .katex-display {
  font-size: 1.6rem;
  overflow-x: auto;
  padding: 0.25rem 0;
}

/* Search results */
.fb-search-results {}


.fb-search-results .fb-grid-header {
  margin-bottom: 1rem;
}

/* No results */
.fb-empty {
  text-align: center;
  color: var(--text-secondary, #6b7280);
  padding: 3rem 1rem;
  font-size: 0.9rem;
}

/* ── Dark mode ───────────────────────────────────────── */
[data-theme="dark"] .fb-header {
  background: var(--bg-primary, #1a1a2e);
  border-bottom-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-sidebar {
  background: var(--bg-secondary, #16213e);
  border-right-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-topic-btn:hover {
  background: rgba(255,255,255,0.05);
}

[data-theme="dark"] .fb-card {
  background: var(--bg-secondary, #16213e);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-card:hover {
  border-color: var(--color-primary, #818cf8);
}

[data-theme="dark"] .fb-formula-hero {
  background: rgba(255,255,255,0.04);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-conditions {
  background: rgba(255,255,255,0.03);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-section {
  background: var(--bg-secondary, #16213e);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-exercise-card {
  background: rgba(99,102,241,0.06);
  border-color: rgba(99,102,241,0.25);
}

[data-theme="dark"] .fb-exercise-input {
  background: var(--bg-primary, #1a1a2e);
  border-color: var(--border-color, #2d2d44);
  color: var(--text-primary, #e2e8f0);
}

[data-theme="dark"] .fb-anim-panel {
  background: #020617;
}

[data-theme="dark"] .fb-derives-section {
  background: rgba(255,255,255,0.03);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-derive-card {
  background: var(--bg-primary, #1a1a2e);
  border-color: var(--border-color, #2d2d44);
}

[data-theme="dark"] .fb-derive-f {
  background: rgba(255,255,255,0.06);
}

/* ── Responsive ──────────────────────────────────────── */
@media (max-width: 1100px) {
  .fb-detail-grid {
    grid-template-columns: 1fr 300px;
  }
}

@media (max-width: 900px) {
  .fb-layout {
    grid-template-columns: 1fr;
  }

  .fb-sidebar {
    position: static;
    height: auto;
    border-right: none;
    border-bottom: 1px solid var(--border-color, #e5e7eb);
    overflow-x: auto;
    overflow-y: hidden;
  }

  .fb-sidebar-inner {
    display: flex;
    flex-direction: row;
    padding: 0.5rem 0.75rem;
    gap: 0.25rem;
    width: max-content;
  }

  .fb-topic-btn {
    border-left: none;
    border-bottom: 3px solid transparent;
    padding: 0.4rem 0.75rem;
    font-size: 0.75rem;
    flex-direction: column;
    align-items: center;
    gap: 0.15rem;
    min-width: 68px;
    text-align: center;
  }

  .fb-topic-btn.active {
    border-left-color: transparent;
    border-bottom-color: var(--color-primary, #6366f1);
  }

  .fb-topic-count { display: none; }

  .fb-detail-grid {
    grid-template-columns: 1fr;
  }

  .fb-two-col {
    grid-template-columns: 200px 1fr;
  }

  .fb-search-wrap {
    flex: 0 0 100%;
  }

  .fb-header {
    padding: 1rem;
  }
}

@media (max-width: 640px) {
  .fb-main {
    padding: 1rem;
  }

  .fb-grid {
    grid-template-columns: repeat(auto-fill, minmax(145px, 1fr));
  }

  .fb-two-col {
    grid-template-columns: 1fr;
  }

  .fb-anim-panel {
    aspect-ratio: 16/9;
  }

  .fb-detail-name {
    font-size: 1.15rem;
  }
}