:root {
  --bg: #0a0a0f;
  --card-bg: rgba(18,18,30,0.75);
  --card-border: rgba(255,255,255,0.06);
  --text: #e0e0e0;
  --text-dim: #b0b0c0;
  --text-muted: #94a3b8;
  --text-faint: #64748b;
  --accent: #38bdf8;
  --accent-glow: rgba(56,189,248,0.15);
  --accent-border: rgba(56,189,248,0.2);
  --accent-dim: rgba(56,189,248,0.12);
  --highlight: #facc15;
  --highlight-bg: rgba(250,204,21,0.1);
  --highlight-border: rgba(250,204,21,0.2);
  --success: #22c55e;
  --success-bg: rgba(34,197,94,0.1);
  --success-border: rgba(34,197,94,0.4);
  --danger: #f87171;
  --danger-bg: rgba(239,68,68,0.1);
  --danger-border: rgba(239,68,68,0.4);
  --warn-bg: rgba(239,68,68,0.08);
  --warn-border: rgba(239,68,68,0.15);
  --warn-text: #fca5a5;
  --tip-bg: rgba(34,197,94,0.08);
  --tip-text: #86efac;
  --nav-bg: rgba(10,10,15,0.85);
  --tag-def: rgba(56,189,248,0.12);
  --tag-adv: rgba(34,197,94,0.12);
  --tag-pair: rgba(250,204,21,0.12);
  --tag-prop: rgba(168,85,247,0.12);
  --tag-ivt: rgba(239,68,68,0.12);
  --tag-gen: rgba(56,189,248,0.12);
  --formula-bg: rgba(0,0,0,0.4);
  --formula-border: rgba(250,204,21,0.1);
  --table-header: rgba(0,0,0,0.3);
  --opt-bg: rgba(255,255,255,0.03);
}

[data-theme="lec2"] {
  --bg: #0f0a06;
  --card-bg: rgba(25, 18, 12, 0.75);
  --card-border: rgba(255,200,130,0.06);
  --text: #f0e8dc;
  --text-dim: #d8ccba;
  --text-muted: #b8a890;
  --text-faint: #8a7a68;
  --accent: #fbbf24;
  --accent-glow: rgba(251,191,36,0.12);
  --accent-border: rgba(251,191,36,0.2);
  --accent-dim: rgba(251,191,36,0.1);
  --highlight: #fde68a;
  --highlight-bg: rgba(253,230,138,0.1);
  --highlight-border: rgba(253,230,138,0.2);
  --tag-def: rgba(251,191,36,0.12);
  --tag-adv: rgba(253,230,138,0.12);
  --tag-pair: rgba(248,113,113,0.12);
  --tag-prop: rgba(168,85,247,0.12);
  --tag-ivt: rgba(239,68,68,0.12);
  --tag-gen: rgba(251,191,36,0.12);
  --formula-bg: rgba(0,0,0,0.4);
  --formula-border: rgba(251,191,36,0.1);
  --table-header: rgba(0,0,0,0.3);
  --opt-bg: rgba(255,200,130,0.03);
  --nav-bg: rgba(15,10,6,0.85);
  --warn-bg: rgba(239,68,68,0.08);
  --warn-border: rgba(239,68,68,0.15);
  --warn-text: #fca5a5;
  --tip-bg: rgba(253,230,138,0.08);
  --tip-text: #fde68a;
  --success: #34d399;
  --success-bg: rgba(52,211,153,0.1);
  --success-border: rgba(52,211,153,0.4);
}

[data-theme="lec3"] {
  --bg: #060d0f;
  --card-bg: rgba(12, 25, 28, 0.75);
  --card-border: rgba(94,234,212,0.06);
  --text: #ccfbf1;
  --text-dim: #a8e6d8;
  --text-muted: #7fc9ba;
  --text-faint: #5a9a8a;
  --accent: #2dd4bf;
  --accent-glow: rgba(45,212,191,0.12);
  --accent-border: rgba(45,212,191,0.2);
  --accent-dim: rgba(45,212,191,0.1);
  --highlight: #5eead4;
  --highlight-bg: rgba(94,234,212,0.1);
  --highlight-border: rgba(94,234,212,0.2);
  --tag-def: rgba(45,212,191,0.12);
  --tag-adv: rgba(94,234,212,0.12);
  --tag-pair: rgba(244,114,182,0.12);
  --tag-prop: rgba(196,181,253,0.12);
  --tag-ivt: rgba(239,68,68,0.12);
  --tag-gen: rgba(45,212,191,0.12);
  --formula-bg: rgba(0,0,0,0.4);
  --formula-border: rgba(94,234,212,0.1);
  --table-header: rgba(0,0,0,0.3);
  --opt-bg: rgba(94,234,212,0.03);
  --nav-bg: rgba(6,13,15,0.85);
  --warn-bg: rgba(239,68,68,0.08);
  --warn-border: rgba(239,68,68,0.15);
  --warn-text: #fca5a5;
  --tip-bg: rgba(94,234,212,0.08);
  --tip-text: #5eead4;
  --success: #22d3ee;
  --success-bg: rgba(34,211,238,0.1);
  --success-border: rgba(34,211,238,0.4);
}

* { box-sizing: border-box; margin: 0; padding: 0; }

body {
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  padding: 0;
  min-height: 100vh;
  transition: background 0.6s ease, color 0.6s ease;
}

body::before {
  content: '';
  position: fixed;
  top: -50%;
  left: -50%;
  width: 200%;
  height: 200%;
  background: radial-gradient(ellipse at 30% 20%, var(--accent-glow) 0%, transparent 50%),
              radial-gradient(ellipse at 70% 80%, rgba(168, 85, 247, 0.05) 0%, transparent 50%);
  z-index: -1;
  pointer-events: none;
  transition: background 0.6s ease;
}

.container { max-width: 480px; margin: 0 auto; padding: 12px 12px 24px; }

/* Nav */
.nav {
  position: sticky;
  top: 0;
  z-index: 100;
  background: var(--nav-bg);
  backdrop-filter: blur(24px);
  -webkit-backdrop-filter: blur(24px);
  border-bottom: 1px solid var(--card-border);
  transition: background 0.6s ease, border-color 0.6s ease;
}
.nav-inner {
  max-width: 480px;
  margin: 0 auto;
  display: flex;
  padding: 0 12px;
}
.nav-btn {
  flex: 1;
  padding: 14px 8px 12px;
  background: none;
  border: none;
  color: var(--text-faint);
  font-size: 0.8rem;
  font-weight: 600;
  cursor: pointer;
  border-bottom: 2px solid transparent;
  transition: all 0.3s ease;
  -webkit-tap-highlight-color: transparent;
  text-transform: uppercase;
  letter-spacing: 1px;
}
.nav-btn:active { transform: scale(0.96); }
.nav-btn.active {
  color: var(--accent);
  border-bottom-color: var(--accent);
}

/* Header */
h1 {
  font-size: 1.6rem;
  font-weight: 700;
  color: #fff;
  text-align: center;
  letter-spacing: -0.3px;
  transition: color 0.6s ease;
}
h1 small {
  display: block;
  font-size: 0.8rem;
  font-weight: 400;
  color: var(--text-faint);
  margin-top: 2px;
  transition: color 0.6s ease;
}
.credit {
  text-align: center;
  font-size: 0.7rem;
  color: var(--text-faint);
  margin-bottom: 16px;
  padding-bottom: 12px;
  border-bottom: 1px solid var(--card-border);
  transition: color 0.6s ease, border-color 0.6s ease;
}
.credit a { color: var(--accent); text-decoration: none; transition: color 0.6s ease; }

/* Cards */
.card {
  background: var(--card-bg);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border-radius: 18px;
  padding: 20px;
  margin-bottom: 16px;
  border: 1px solid var(--card-border);
  box-shadow: 0 8px 32px rgba(0,0,0,0.3);
  transition: background 0.6s ease, border-color 0.6s ease;
}
.card-title {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 1.5px;
  color: var(--accent);
  margin-bottom: 12px;
  display: flex;
  align-items: center;
  gap: 8px;
  transition: color 0.6s ease;
}
.card-title .badge {
  font-size: 0.6rem;
  padding: 2px 8px;
  border-radius: 10px;
  font-weight: 600;
  text-transform: none;
  letter-spacing: 0;
}
.badge-accent { background: var(--accent-dim); color: var(--accent); }
.badge-highlight { background: var(--highlight-bg); color: var(--highlight); }
.badge-danger { background: var(--danger-bg); color: var(--danger); }
.badge-prop { background: var(--tag-prop); color: #c084fc; }

.card p, .card li { font-size: 0.88rem; color: var(--text-dim); transition: color 0.6s ease; }
.card ul, .card ol { padding-left: 18px; margin-top: 6px; }
.card li { margin-bottom: 4px; }
.card strong { color: #fff; transition: color 0.6s ease; }

/* Reminder card */
.reminder-card {
  background: linear-gradient(135deg, rgba(239,68,68,0.06), rgba(239,68,68,0.03));
  border: 1px solid rgba(239,68,68,0.15);
  border-radius: 18px;
  padding: 18px 20px;
  margin-bottom: 18px;
  cursor: pointer;
  box-shadow: 0 4px 20px rgba(239,68,68,0.05);
  transition: background 0.6s ease, border-color 0.6s ease;
}
.reminder-card .reminder-title {
  font-size: 0.65rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 2px;
  color: #f87171;
  display: flex;
  align-items: center;
  gap: 8px;
  user-select: none;
}
.reminder-card .reminder-title span { font-size: 1rem; }
.reminder-card .reminder-title::after {
  content: '\25B6';
  margin-left: auto;
  font-size: 0.6rem;
  transition: transform 0.3s ease;
  color: rgba(248,113,113,0.4);
}
.reminder-card.open .reminder-title::after {
  transform: rotate(90deg);
}
.reminder-card .reminder-content {
  max-height: 0;
  opacity: 0;
  overflow: hidden;
  transition: max-height 0.4s ease, opacity 0.4s ease, margin-top 0.4s ease;
  margin-top: 0;
}
.reminder-card.open .reminder-content {
  max-height: 1200px;
  opacity: 1;
  margin-top: 12px;
}
.reminder-card p, .reminder-card li {
  font-size: 0.85rem;
  color: var(--text-dim);
  transition: color 0.6s ease;
}
.reminder-card ul, .reminder-card ol { padding-left: 18px; margin-top: 4px; }
.reminder-card li { margin-bottom: 3px; }
.reminder-card strong { color: #fca5a5; }
.reminder-card code {
  background: rgba(0,0,0,0.3);
  padding: 1px 6px;
  border-radius: 4px;
  font-family: 'Consolas', 'Courier New', monospace;
  font-size: 0.82rem;
  color: #fbbf24;
}
.reminder-card .rm-formula {
  background: rgba(0,0,0,0.25);
  border-radius: 8px;
  padding: 10px 14px;
  margin: 8px 0;
  font-family: 'Consolas', 'Courier New', monospace;
  font-size: 0.88rem;
  color: #fbbf24;
  text-align: center;
  border: 1px solid rgba(251,191,36,0.1);
}

.formula {
  background: var(--formula-bg);
  border-radius: 12px;
  padding: 14px;
  margin: 10px 0;
  font-family: 'Consolas', 'Courier New', monospace;
  font-size: 1rem;
  color: var(--highlight);
  text-align: center;
  border: 1px solid var(--formula-border);
  transition: background 0.6s ease, border-color 0.6s ease, color 0.6s ease;
}
.formula .lbl {
  font-size: 0.65rem;
  color: var(--text-faint);
  font-family: 'Segoe UI', sans-serif;
  display: block;
  margin-bottom: 4px;
  text-transform: uppercase;
  letter-spacing: 1px;
  transition: color 0.6s ease;
}
.highlight {
  background: var(--highlight-bg);
  border: 1px solid var(--highlight-border);
  border-radius: 12px;
  padding: 14px;
  margin: 12px 0;
  color: var(--highlight);
  font-weight: 600;
  text-align: center;
  font-size: 0.88rem;
  transition: background 0.6s ease, border-color 0.6s ease, color 0.6s ease;
}
.warn {
  background: var(--warn-bg);
  border: 1px solid var(--warn-border);
  border-radius: 12px;
  padding: 12px 14px;
  margin: 10px 0;
  color: var(--warn-text);
  font-size: 0.82rem;
  transition: background 0.6s ease, border-color 0.6s ease, color 0.6s ease;
}
.tip {
  background: var(--tip-bg);
  border-left: 3px solid var(--success);
  border-radius: 0 10px 10px 0;
  padding: 10px 14px;
  margin: 10px 0;
  color: var(--tip-text);
  font-size: 0.82rem;
  transition: background 0.6s ease, color 0.6s ease;
}
table {
  width: 100%;
  border-collapse: collapse;
  margin: 10px 0;
  font-size: 0.8rem;
  display: block;
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
}
th, td {
  padding: 8px 10px;
  text-align: left;
  border-bottom: 1px solid var(--card-border);
  white-space: nowrap;
  transition: border-color 0.6s ease, color 0.6s ease;
}
th { background: var(--table-header); color: var(--accent); font-weight: 600; font-size: 0.75rem; text-transform: uppercase; letter-spacing: 0.5px; transition: background 0.6s ease, color 0.6s ease; }
tr:hover td { background: var(--opt-bg); }

/* MCQs */
.mcq-card {
  background: var(--card-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border-radius: 18px;
  margin-bottom: 14px;
  border: 1px solid var(--card-border);
  overflow: hidden;
  transition: background 0.6s ease, border-color 0.6s ease;
}
.mcq-card .q-header {
  padding: 16px 18px 10px;
  display: flex;
  align-items: flex-start;
  gap: 10px;
}
.mcq-card .q-num {
  background: var(--accent-dim);
  color: var(--accent);
  border-radius: 10px;
  min-width: 30px;
  height: 30px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 700;
  font-size: 0.8rem;
  flex-shrink: 0;
  transition: background 0.6s ease, color 0.6s ease;
}
.mcq-card .q-text { font-weight: 500; font-size: 0.9rem; color: var(--text); padding-top: 4px; transition: color 0.6s ease; }
.mcq-card .q-tags { display: flex; gap: 4px; flex-wrap: wrap; margin-bottom: 4px; }
.mcq-card .opts { padding: 6px 18px 14px; display: flex; flex-direction: column; gap: 6px; }
.mcq-card .opts label {
  display: flex; align-items: center; gap: 10px;
  padding: 10px 12px;
  background: var(--opt-bg);
  border-radius: 12px;
  cursor: pointer;
  transition: all 0.15s;
  border: 1.5px solid transparent;
  font-size: 0.85rem;
  color: var(--text-dim);
  -webkit-tap-highlight-color: transparent;
}
.mcq-card .opts label:hover { background: rgba(255,255,255,0.06); }
.mcq-card .opts input[type="radio"] { accent-color: var(--accent); width: 16px; height: 16px; flex-shrink: 0; }
.mcq-card .opts .correct { border-color: var(--success-border) !important; background: var(--success-bg) !important; color: var(--success) !important; }
.mcq-card .opts .wrong { border-color: var(--danger-border) !important; background: var(--danger-bg) !important; color: var(--danger) !important; }
.mcq-card .explain { padding: 0 18px 18px; display: none; }
.mcq-card .explain.show { display: block; }
.mcq-card .explain-inner {
  background: var(--formula-bg);
  border-radius: 12px;
  padding: 12px 14px;
  font-size: 0.82rem;
  color: var(--text-muted);
  border: 1px solid var(--card-border);
  transition: background 0.6s ease, border-color 0.6s ease, color 0.6s ease;
}
.mcq-card .explain-inner strong { color: var(--highlight); transition: color 0.6s ease; }

.tag {
  display: inline-block;
  padding: 2px 8px;
  border-radius: 8px;
  font-size: 0.6rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.5px;
}
.tag-intro { background: var(--tag-def); color: var(--accent); }
.tag-adv { background: var(--tag-adv); color: var(--success); }
.tag-def { background: var(--tag-def); color: var(--accent); }
.tag-pair { background: var(--tag-pair); color: var(--highlight); }
.tag-prop { background: var(--tag-prop); color: #c084fc; }
.tag-ivt { background: var(--tag-ivt); color: var(--danger); }
.tag-gen { background: var(--tag-gen); color: var(--accent); }

/* Score bar */
.score-bar {
  background: var(--card-bg);
  backdrop-filter: blur(24px);
  -webkit-backdrop-filter: blur(24px);
  border-radius: 18px;
  padding: 18px;
  text-align: center;
  margin-bottom: 16px;
  border: 1px solid var(--card-border);
  transition: background 0.6s ease, border-color 0.6s ease;
}
.score-bar .big { font-size: 2.2rem; font-weight: 800; color: var(--accent); transition: color 0.6s ease; }
.score-bar .lbl { font-size: 0.7rem; text-transform: uppercase; letter-spacing: 1.5px; color: var(--text-faint); }
.score-bar .pct { font-size: 0.75rem; color: var(--text-faint); margin-top: 2px; }
.progress-wrap { height: 3px; background: var(--opt-bg); border-radius: 4px; margin-top: 10px; overflow: hidden; }
.progress-wrap .fill { height: 100%; border-radius: 4px; background: linear-gradient(90deg, var(--accent), var(--success)); transition: width 0.4s ease; }

/* Filter */
.filter-bar {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
  margin-bottom: 14px;
  overflow-x: auto;
  -webkit-overflow-scrolling: touch;
  padding-bottom: 4px;
}
.filter-bar button {
  padding: 6px 14px;
  border-radius: 20px;
  border: 1px solid var(--card-border);
  background: var(--card-bg);
  backdrop-filter: blur(8px);
  color: var(--text-faint);
  cursor: pointer;
  font-size: 0.72rem;
  font-weight: 600;
  transition: all 0.15s;
  white-space: nowrap;
  -webkit-tap-highlight-color: transparent;
}
.filter-bar button.active { background: var(--accent-dim); color: var(--accent); border-color: var(--accent-border); }

/* Buttons */
.btn-group {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  justify-content: center;
  margin: 18px 0 10px;
}
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: 6px;
  padding: 10px 20px;
  border-radius: 14px;
  border: none;
  font-size: 0.82rem;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.15s;
  -webkit-tap-highlight-color: transparent;
  flex: 1;
  min-width: 0;
}
.btn:active { transform: scale(0.96); }
.btn-primary { background: var(--accent-dim); color: var(--accent); border: 1px solid var(--accent-border); }
.btn-warning { background: var(--highlight-bg); color: var(--highlight); border: 1px solid var(--highlight-border); }
.btn-danger { background: var(--danger-bg); color: var(--danger); border: 1px solid var(--danger-border); }
.btn-outline { background: var(--opt-bg); color: var(--text-faint); border: 1px solid var(--card-border); }

/* Lec3 placeholder */
.placeholder-card {
  background: var(--card-bg);
  backdrop-filter: blur(20px);
  border-radius: 18px;
  padding: 48px 20px;
  text-align: center;
  border: 1px solid var(--card-border);
}
.placeholder-card .big-emoji { font-size: 3rem; margin-bottom: 12px; }
.placeholder-card p { color: var(--text-faint); font-size: 0.9rem; }

/* Lecture content toggle */
.lec-content { display: none; }
.lec-content.active { display: block; }

/* Theme switch animation */
.theme-switching {
  animation: themePulse 0.6s ease;
}
@keyframes themePulse {
  0% { opacity: 1; }
  30% { opacity: 0.4; filter: blur(2px); }
  60% { opacity: 0.4; filter: blur(2px); }
  100% { opacity: 1; filter: blur(0); }
}

@media (max-width: 380px) {
  h1 { font-size: 1.3rem; }
  .card { padding: 16px; }
  .score-bar .big { font-size: 1.8rem; }
}