:root {
  --bg: #0f1216;
  --bg-panel: #171b21;
  --bg-raised: #1e232b;
  --border: #2a313c;
  --text: #e8ecf1;
  --text-dim: #9aa5b1;
  --accent: #7dd3fc;
  --accent-2: #f2a65a;
  --accent-3: #7ee787;
  --accent-4: #f77;
  --danger: #ff6b6b;
  --radius: 10px;
  --sidebar-w: 240px;
  font-size: 16px;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.6;
}

a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

.layout {
  display: flex;
  min-height: 100vh;
}

.sidebar {
  width: var(--sidebar-w);
  flex-shrink: 0;
  background: var(--bg-panel);
  border-right: 1px solid var(--border);
  padding: 1.5rem 1rem;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.sidebar .brand {
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--text);
  display: block;
  margin-bottom: 1.5rem;
}

.sidebar .brand span { color: var(--accent); }

.sidebar nav ol {
  list-style: none;
  margin: 0;
  padding: 0;
  counter-reset: chapter;
}

.sidebar nav li {
  margin-bottom: 0.15rem;
}

.sidebar nav a {
  display: block;
  padding: 0.5rem 0.6rem;
  border-radius: 8px;
  color: var(--text-dim);
  font-size: 0.92rem;
}

.sidebar nav a:hover {
  background: var(--bg-raised);
  text-decoration: none;
  color: var(--text);
}

.sidebar nav a.active {
  background: var(--bg-raised);
  color: var(--accent);
  font-weight: 600;
}

.sidebar nav .step-num {
  display: inline-block;
  width: 1.4em;
  color: var(--text-dim);
  font-variant-numeric: tabular-nums;
}

main {
  flex: 1;
  min-width: 0;
  padding: 2.5rem 3rem 6rem;
  max-width: 880px;
}

h1 {
  font-size: 2rem;
  margin: 0 0 0.3rem;
}

h1 .kicker {
  display: block;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--accent);
  font-weight: 600;
  margin-bottom: 0.5rem;
}

h2 {
  font-size: 1.35rem;
  margin: 2.5rem 0 0.8rem;
  padding-top: 0.5rem;
  border-top: 1px solid var(--border);
}

h2:first-of-type { border-top: none; padding-top: 0; }

p { color: var(--text); }
p.lead {
  font-size: 1.1rem;
  color: var(--text-dim);
  margin-top: 0.5rem;
}

.callout {
  background: var(--bg-panel);
  border: 1px solid var(--border);
  border-left: 3px solid var(--accent);
  border-radius: var(--radius);
  padding: 1rem 1.2rem;
  margin: 1.2rem 0;
  font-size: 0.95rem;
  color: var(--text-dim);
}

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

.formula-box {
  background: var(--bg-raised);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.1rem 1.4rem;
  margin: 1.2rem 0;
  overflow-x: auto;
}

.formula-box .katex-display { margin: 0.3rem 0; }

.formula-label {
  font-size: 0.78rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-dim);
  margin-bottom: 0.4rem;
}

.viz {
  background: var(--bg-panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.2rem;
  margin: 1.2rem 0 1.6rem;
}

.viz canvas {
  display: block;
  width: 100%;
  background: #0a0d11;
  border-radius: 6px;
  touch-action: none;
}

.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 1.4rem 2rem;
  margin-top: 1rem;
  padding-top: 1rem;
  border-top: 1px solid var(--border);
  align-items: flex-start;
}

.control {
  display: flex;
  flex-direction: column;
  gap: 0.3rem;
  min-width: 140px;
  flex: 1 1 160px;
}

.control label {
  font-size: 0.82rem;
  color: var(--text-dim);
  display: flex;
  justify-content: space-between;
}

.control label .val {
  color: var(--accent);
  font-variant-numeric: tabular-nums;
  font-weight: 600;
}

/* Rotary knob: drag vertically or scroll to change. */
.knob-widget {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.3rem;
  width: 84px;
  user-select: none;
}

.knob-widget svg {
  width: 60px;
  height: 60px;
  cursor: ns-resize;
  touch-action: none;
  overflow: visible;
}

.knob-widget svg.dragging { cursor: grabbing; }

.knob-track {
  fill: none;
  stroke: var(--border);
  stroke-width: 5;
}

.knob-arc {
  fill: none;
  stroke: var(--accent);
  stroke-width: 5;
  stroke-linecap: round;
  transition: stroke-dasharray 0.02s linear;
}

.knob-cap {
  fill: var(--bg-raised);
  stroke: var(--border);
  stroke-width: 1;
}

.knob-pointer {
  stroke: var(--text);
  stroke-width: 2.5;
  stroke-linecap: round;
  transform-origin: 32px 32px;
  transition: transform 0.02s linear;
}

.knob-label {
  font-size: 0.78rem;
  color: var(--text-dim);
  text-align: center;
}

.knob-value {
  font-size: 0.85rem;
  color: var(--accent);
  font-weight: 600;
  font-variant-numeric: tabular-nums;
}

/* Drag-to-scrub number, for integer counts. */
.dragnum {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.4rem;
  user-select: none;
}

.dragnum-label {
  font-size: 0.82rem;
  color: var(--text-dim);
}

.dragnum-display {
  display: flex;
  align-items: center;
  gap: 0.5rem;
}

.dragnum-value {
  font-size: 1.5rem;
  font-weight: 700;
  color: var(--text);
  font-variant-numeric: tabular-nums;
  min-width: 2.6em;
  text-align: center;
  padding: 0.15rem 0.5rem;
  border-radius: 8px;
  background: var(--bg-raised);
  border: 1px solid var(--border);
  cursor: ns-resize;
}

.dragnum-value:hover, .dragnum-value.dragging {
  border-color: var(--accent);
  color: var(--accent);
}

.dragnum-btn {
  background: var(--bg-raised);
  border: 1px solid var(--border);
  color: var(--text-dim);
  width: 30px;
  height: 30px;
  border-radius: 7px;
  cursor: pointer;
  font-size: 1.1rem;
  line-height: 1;
  display: flex;
  align-items: center;
  justify-content: center;
}

.dragnum-btn:hover { border-color: var(--accent); color: var(--accent); }

.dial-row {
  display: flex;
  gap: 1.5rem;
  flex-wrap: wrap;
  margin-bottom: 1rem;
}

.dial-card {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 0.6rem;
  background: var(--bg-raised);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1rem;
  flex: 1 1 260px;
}

.dial-card canvas {
  width: 100%;
  background: #0a0d11;
  border-radius: 6px;
}

.dial-card .dial-label {
  font-size: 0.85rem;
  font-weight: 700;
}

/* Hint that a canvas has a directly-draggable element on it. */
.drag-hint {
  font-size: 0.78rem;
  color: var(--text-dim);
  margin-top: 0.5rem;
}

.drag-hint strong { color: var(--text); }

/* Small grid-of-pixels canvases (2D FFT chapter): fixed low internal
   resolution, shown blocky-crisp rather than blurred, to keep the "an
   image is a grid of numbers" idea visible. */
.image-row {
  display: flex;
  gap: 1.2rem;
  flex-wrap: wrap;
  align-items: flex-start;
}

.image-cell {
  display: flex;
  flex-direction: column;
  gap: 0.5rem;
  flex: 1 1 160px;
  max-width: 220px;
}

.image-cell .cell-label {
  font-size: 0.78rem;
  color: var(--text-dim);
  text-align: center;
}

canvas.pixel-canvas {
  width: 100%;
  aspect-ratio: 1;
  image-rendering: pixelated;
  cursor: default;
}

button.btn, select.btn {
  background: var(--bg-raised);
  border: 1px solid var(--border);
  color: var(--text);
  padding: 0.45rem 0.9rem;
  border-radius: 8px;
  cursor: pointer;
  font-size: 0.88rem;
}

button.btn:hover { border-color: var(--accent); color: var(--accent); }
button.btn.active { border-color: var(--accent); color: var(--accent); background: #10202a; }

.legend {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  font-size: 0.82rem;
  color: var(--text-dim);
  margin-top: 0.6rem;
}

.legend .swatch {
  display: inline-block;
  width: 0.8em;
  height: 0.8em;
  border-radius: 2px;
  margin-right: 0.4em;
  vertical-align: middle;
}

.pager {
  display: flex;
  justify-content: space-between;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  font-size: 0.95rem;
}

.pager a {
  color: var(--text);
  display: flex;
  flex-direction: column;
}

.pager a .dir {
  font-size: 0.75rem;
  color: var(--text-dim);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.pager .next { text-align: right; margin-left: auto; }

code, .mono {
  font-family: "SF Mono", Menlo, Consolas, monospace;
  font-size: 0.9em;
  background: var(--bg-raised);
  padding: 0.1em 0.4em;
  border-radius: 4px;
}

.toc-grid {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  margin-top: 1.5rem;
}

@media (max-width: 700px) {
  .toc-grid { grid-template-columns: 1fr; }
}

.toc-card {
  background: var(--bg-panel);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1rem 1.2rem;
  display: block;
  color: var(--text);
}

.toc-card:hover {
  border-color: var(--accent);
  text-decoration: none;
}

.toc-card .num {
  font-size: 0.78rem;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: 0.05em;
}

.toc-card h3 { margin: 0.3rem 0 0.3rem; font-size: 1.05rem; }
.toc-card p { margin: 0; font-size: 0.88rem; color: var(--text-dim); }

@media (max-width: 800px) {
  .layout { flex-direction: column; }
  .sidebar { width: 100%; height: auto; position: static; }
  main { padding: 1.5rem; max-width: 100%; }
}