:root {
  --bg: #0a0a0b;
  --bg-elevated: #111113;
  --border: #232326;
  --border-hover: #35353a;
  --text: #ededed;
  --text-dim: #8a8a92;
  --text-faint: #55555c;
  --accent-nt: #6c8fff;   /* number theory — indigo */
  --accent-trig: #ff8a5c; /* trigonometry — amber-coral */
  --accent-calc: #7ee08c; /* calculus — mint green */
  --accent-comb: #d68aff; /* combinatorics — violet */
  --accent-alg: #ffd166;  /* algebra — gold */
  --mono: 'JetBrains Mono', ui-monospace, monospace;
  --sans: 'Inter', -apple-system, sans-serif;
}

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

html { scroll-behavior: smooth; }

body {
  background: var(--bg);
  color: var(--text);
  font-family: var(--sans);
  -webkit-font-smoothing: antialiased;
  line-height: 1.5;
}

::selection { background: var(--accent-nt); color: #0a0a0b; }

a { color: inherit; }

/* ---------- NAV ---------- */
nav {
  position: sticky;
  top: 0;
  z-index: 50;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 32px;
  background: rgba(10,10,11,0.8);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}

.logo {
  font-family: var(--mono);
  font-size: 14px;
  font-weight: 600;
  letter-spacing: -0.02em;
  display: flex;
  align-items: center;
  gap: 8px;
}
.logo-mark {
  width: 20px; height: 20px;
  border: 1.5px solid var(--text);
  border-radius: 4px;
  display: flex; align-items: center; justify-content: center;
  font-size: 11px;
  font-weight: 700;
}

.logo-group {
  display: flex;
  align-items: baseline;
  gap: 10px;
}
.signature {
  font-family: 'Impact', 'Haettenschweiler', 'Arial Narrow Bold', sans-serif;
  font-size: 17px;
  font-weight: 400;
  color: var(--text);
  letter-spacing: 0.02em;
  padding-left: 10px;
  border-left: 1px solid var(--border);
  line-height: 1;
  text-transform: uppercase;
}

.nav-links {
  display: flex;
  gap: 28px;
  font-size: 13px;
  color: var(--text-dim);
}
.nav-links a { text-decoration: none; transition: color 0.15s; }
.nav-links a:hover, .nav-links a.active { color: var(--text); }

.menu-toggle {
  display: none;
  background: none;
  border: 1px solid var(--border);
  border-radius: 6px;
  padding: 8px;
  color: var(--text);
  cursor: pointer;
}

/* ---------- HERO ---------- */
.hero {
  position: relative;
  padding: 96px 32px 72px;
  max-width: 1040px;
  margin: 0 auto;
  overflow: hidden;
}

.hero-canvas {
  position: absolute;
  top: -40px; right: -60px;
  width: 480px; height: 480px;
  opacity: 0.5;
  pointer-events: none;
}

.eyebrow {
  font-family: var(--mono);
  font-size: 12px;
  color: var(--text-faint);
  letter-spacing: 0.06em;
  margin-bottom: 20px;
  display: flex;
  align-items: center;
  gap: 10px;
}
.eyebrow::before {
  content: '';
  width: 6px; height: 6px;
  border-radius: 50%;
  background: var(--accent-eyebrow, var(--accent-nt));
  box-shadow: 0 0 8px var(--accent-eyebrow, var(--accent-nt));
}

h1 {
  font-size: clamp(38px, 6vw, 62px);
  font-weight: 800;
  letter-spacing: -0.035em;
  line-height: 1.04;
  max-width: 680px;
}
h1 .accent { color: var(--accent-eyebrow, var(--accent-nt)); }

.hero p {
  margin-top: 20px;
  font-size: 17px;
  color: var(--text-dim);
  max-width: 480px;
  line-height: 1.6;
}

.hero-stats {
  display: flex;
  gap: 40px;
  margin-top: 44px;
}
.stat-num {
  font-family: var(--mono);
  font-size: 26px;
  font-weight: 700;
}
.stat-label {
  font-size: 12px;
  color: var(--text-faint);
  margin-top: 4px;
}

.back-link {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-family: var(--mono);
  font-size: 12.5px;
  color: var(--text-faint);
  text-decoration: none;
  margin-bottom: 28px;
  transition: color 0.15s;
}
.back-link:hover { color: var(--text); }

/* ---------- TOPIC CARDS (landing page) ---------- */
.topic-grid {
  max-width: 1040px;
  margin: 0 auto;
  padding: 8px 32px 100px;
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
  gap: 16px;
}
.topic-card {
  display: block;
  text-decoration: none;
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 24px;
  transition: border-color 0.2s, box-shadow 0.2s, transform 0.2s;
}
.topic-card:hover {
  transform: translateY(-2px);
  border-color: var(--card-accent, var(--border-hover));
  box-shadow: 0 0 0 1px var(--card-accent, var(--border-hover)), 0 12px 32px -12px var(--card-accent, transparent);
}
.topic-card .topic-icon { margin-bottom: 18px; width: 36px; height: 36px; }
.topic-card .topic-icon svg { width: 18px; height: 18px; }
.topic-card h3 { font-size: 17px; font-weight: 700; letter-spacing: -0.01em; margin-bottom: 6px; }
.topic-card p {
  font-size: 13px;
  color: var(--text-faint);
  font-family: var(--mono);
}

/* ---------- SEARCH / FILTER BAR ---------- */
.controls {
  max-width: 1040px;
  margin: 0 auto;
  padding: 0 32px 28px;
  display: flex;
  gap: 12px;
  flex-wrap: wrap;
  align-items: center;
}

.search-box {
  flex: 1;
  min-width: 220px;
  position: relative;
}
.search-box input {
  width: 100%;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 11px 14px 11px 38px;
  color: var(--text);
  font-size: 14px;
  font-family: var(--sans);
  outline: none;
  transition: border-color 0.15s;
}
.search-box input:focus { border-color: var(--border-hover); }
.search-box input::placeholder { color: var(--text-faint); }
.search-icon {
  position: absolute;
  left: 13px; top: 50%;
  transform: translateY(-50%);
  color: var(--text-faint);
  width: 15px; height: 15px;
}

.filter-pills {
  display: flex;
  gap: 8px;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 3px;
}
.pill {
  font-family: var(--mono);
  font-size: 12.5px;
  padding: 7px 14px;
  border-radius: 6px;
  cursor: pointer;
  color: var(--text-dim);
  background: transparent;
  border: none;
  transition: all 0.15s;
}
.pill.active { background: #1c1c1f; color: var(--text); }
.pill:hover:not(.active) { color: var(--text); }

/* ---------- SECTIONS / QUESTIONS ---------- */
main { max-width: 1040px; margin: 0 auto; padding: 8px 32px 100px; }

.topic-section { margin-top: 16px; }

.topic-header {
  display: flex;
  align-items: baseline;
  gap: 14px;
  margin-bottom: 6px;
  padding-bottom: 16px;
  border-bottom: 1px solid var(--border);
}
.topic-icon {
  width: 30px; height: 30px;
  border-radius: 7px;
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
}
.topic-icon svg { width: 16px; height: 16px; }
.topic-header h2 { font-size: 20px; font-weight: 700; letter-spacing: -0.02em; }
.topic-count {
  font-family: var(--mono);
  font-size: 12px;
  color: var(--text-faint);
  margin-left: auto;
}

.question-list { margin-top: 4px; }

.q-item {
  border-bottom: 1px solid var(--border);
  padding: 20px 4px;
  cursor: pointer;
  transition: background 0.15s, box-shadow 0.2s, border-color 0.2s;
  border-radius: 8px;
}
.q-item:hover {
  background: rgba(255,255,255,0.02);
  box-shadow: 0 0 0 1px var(--glow, var(--border-hover)), 0 0 24px -8px var(--glow, transparent);
}
.q-item.has-link:hover { background: rgba(108,143,255,0.04); }
.q-item.has-link .chevron { display: none; }
.link-icon {
  color: var(--text-faint);
  flex-shrink: 0;
  padding-top: 4px;
  transition: color 0.15s;
}
.q-item.has-link:hover .link-icon { color: var(--accent-nt); }

.q-row {
  display: flex;
  align-items: flex-start;
  gap: 16px;
}
.q-index {
  font-family: var(--mono);
  font-size: 12px;
  color: var(--text-faint);
  padding-top: 3px;
  width: 24px;
  flex-shrink: 0;
}
.q-body { flex: 1; }
.q-body, .q-answer-inner { min-width: 0; } /* allows children to shrink/scroll instead of overflowing */
.q-text { font-size: 15px; font-weight: 500; line-height: 1.55; overflow-x: auto; }
.katex-display { overflow-x: auto; overflow-y: hidden; padding-bottom: 4px; }
.katex { white-space: normal; }
.q-image {
  max-width: 100%;
  max-height: 260px;
  margin-top: 12px;
  border-radius: 8px;
  border: 1px solid var(--border);
  display: block;
  background: #f4f4f5;
  padding: 12px;
  object-fit: contain;
}
.q-image-linked {
  cursor: pointer;
  transition: border-color 0.15s, opacity 0.15s;
}
.q-image-linked:hover {
  border-color: var(--accent-nt);
  opacity: 0.9;
}
.q-meta { display: flex; gap: 8px; margin-top: 10px; }
.tag {
  font-family: var(--mono);
  font-size: 10.5px;
  padding: 3px 8px;
  border-radius: 4px;
  letter-spacing: 0.02em;
}
.tag-easy { background: rgba(108,255,163,0.1); color: #6cffa3; }
.tag-medium { background: rgba(255,196,92,0.1); color: #ffc45c; }
.tag-hard { background: rgba(255,108,108,0.1); color: #ff6c6c; }

.chevron {
  color: var(--text-faint);
  flex-shrink: 0;
  transition: transform 0.2s;
  padding-top: 4px;
}
.q-item.open .chevron { transform: rotate(180deg); }

.q-answer {
  max-height: 0;
  overflow: hidden;
  transition: max-height 0.25s ease;
}
.q-item.open .q-answer { max-height: 300px; }
.q-answer-inner {
  margin-top: 14px;
  margin-left: 40px;
  padding: 14px 16px;
  background: var(--bg-elevated);
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 14px;
  color: var(--text-dim);
  font-family: var(--mono);
  line-height: 1.6;
}
.q-answer-inner .label {
  color: var(--text-faint);
  font-size: 10.5px;
  letter-spacing: 0.06em;
  display: block;
  margin-bottom: 6px;
}

.empty-state {
  text-align: center;
  padding: 60px 20px;
  color: var(--text-faint);
  font-family: var(--mono);
  font-size: 13px;
}

/* ---------- SKELETON LOADING ---------- */
.skeleton-list { padding-top: 4px; }
.skeleton-item {
  border-bottom: 1px solid var(--border);
  padding: 20px 4px;
  display: flex;
  gap: 16px;
}
.skeleton-index {
  width: 24px; height: 12px;
  border-radius: 3px;
  background: var(--bg-elevated);
  flex-shrink: 0;
  margin-top: 4px;
}
.skeleton-lines { flex: 1; }
.skeleton-line {
  height: 14px;
  border-radius: 4px;
  background: var(--bg-elevated);
  margin-bottom: 10px;
  animation: skeleton-pulse 1.4s ease-in-out infinite;
}
.skeleton-line:nth-child(1) { width: 85%; }
.skeleton-line:nth-child(2) { width: 55%; animation-delay: 0.1s; }
.skeleton-tag {
  width: 60px; height: 18px;
  border-radius: 4px;
  background: var(--bg-elevated);
  margin-top: 4px;
  animation: skeleton-pulse 1.4s ease-in-out infinite;
  animation-delay: 0.2s;
}
.skeleton-index { animation: skeleton-pulse 1.4s ease-in-out infinite; }

/* ---------- CUSTOM CURSOR ---------- */
@media (pointer: fine) {
  body { cursor: none; }
  a, button, .q-item, .topic-card, .pill { cursor: none; }
}
.cursor-glow {
  position: fixed;
  top: 0; left: 0;
  width: 10px; height: 10px;
  border-radius: 50%;
  background: #ffffff;
  box-shadow:
    0 0 8px 2px rgba(255,255,255,0.9),
    0 0 20px 6px rgba(255,255,255,0.35),
    0 0 36px 12px rgba(255,255,255,0.12);
  pointer-events: none;
  z-index: 9999;
  transform: translate(-50%, -50%);
  transition: width 0.15s ease, height 0.15s ease, opacity 0.15s ease;
  opacity: 0;
}
.cursor-glow.visible { opacity: 1; }
.cursor-glow.cursor-hover {
  width: 22px;
  height: 22px;
}

@keyframes skeleton-pulse {
  0%, 100% { opacity: 0.5; }
  50% { opacity: 1; }
}
.skeleton-card {
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 24px;
}
.skeleton-card .skeleton-circle {
  width: 36px; height: 36px;
  border-radius: 9px;
  background: var(--bg-elevated);
  margin-bottom: 18px;
  animation: skeleton-pulse 1.4s ease-in-out infinite;
}
.skeleton-card .skeleton-line {
  animation: skeleton-pulse 1.4s ease-in-out infinite;
}
.skeleton-card .skeleton-line:nth-child(2) { width: 70%; height: 17px; margin-bottom: 8px; }
.skeleton-card .skeleton-line:nth-child(3) { width: 40%; height: 12px; margin-bottom: 0; }

/* ---------- FOOTER ---------- */
footer {
  border-top: 1px solid var(--border);
  padding: 32px;
  text-align: center;
  font-size: 12.5px;
  color: var(--text-faint);
  font-family: var(--mono);
}
.credit {
  margin-top: 10px;
  font-family: -apple-system, BlinkMacSystemFont, 'SF Pro Text', 'Helvetica Neue', sans-serif;
  font-size: 12px;
  font-weight: 500;
  opacity: 0.35;
  letter-spacing: 0.01em;
}

@media (max-width: 640px) {
  nav { padding: 14px 18px; flex-wrap: wrap; }
  .menu-toggle { display: block; }
  .nav-links {
    display: none;
    width: 100%;
    flex-direction: column;
    gap: 0;
    order: 3;
    margin-top: 14px;
    padding-top: 14px;
    border-top: 1px solid var(--border);
  }
  .nav-links.open { display: flex; }
  .nav-links a { padding: 10px 4px; font-size: 14px; }
  .hero { padding: 56px 18px 40px; }
  .controls { padding: 0 18px 24px; }
  main { padding: 8px 18px 80px; }
  .topic-grid { padding: 8px 18px 80px; }
  .hero-stats { gap: 28px; }
  .q-answer-inner { margin-left: 0; }
  .q-row { gap: 10px; }
  .q-index { width: 20px; font-size: 11px; }
  h1 { letter-spacing: -0.02em; }
  .search-box input { font-size: 16px; }
}