:root {
  --ink: #e8f0f4;
  --muted: #8aa0ad;
  --dim: #5c7382;
  --line: rgba(120, 170, 190, 0.18);
  --cyan: #3ec7d6;
  --cyan-dim: #1a7a88;
  --green: #7bc96f;
  --amber: #e0b45a;
  --rose: #e07a7a;
  --bg: #06131c;
  --bg-2: #0a1c28;
  --panel: rgba(10, 28, 40, 0.72);
  --radius: 4px;
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: Onest, "DejaVu Sans", sans-serif;
  color: var(--ink);
  background: var(--bg);
  line-height: 1.55;
  -webkit-font-smoothing: antialiased;
  min-height: 100vh;
  background-image:
    radial-gradient(ellipse 80% 50% at 10% -10%, rgba(62, 199, 214, 0.12), transparent 50%),
    radial-gradient(ellipse 60% 40% at 90% 0%, rgba(123, 201, 111, 0.06), transparent 45%),
    linear-gradient(180deg, var(--bg) 0%, #041018 100%);
}

a {
  color: var(--cyan);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

.mono {
  font-family: "JetBrains Mono", ui-monospace, monospace;
}

.wrap {
  width: min(1120px, calc(100% - 40px));
  margin: 0 auto;
}

/* —— Header —— */
.site-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  padding: 18px 0 8px;
  border-bottom: 1px solid var(--line);
  margin-bottom: 28px;
}

.site-header .mark {
  font-size: 0.95rem;
  letter-spacing: 0.02em;
  color: var(--ink);
}

.site-header .mark span {
  color: var(--cyan);
}

.site-header nav {
  display: flex;
  flex-wrap: wrap;
  gap: 14px 18px;
  font-size: 0.9rem;
}

.site-header nav a {
  color: var(--muted);
}

.site-header nav a:hover,
.site-header nav a[aria-current="page"] {
  color: var(--cyan);
  text-decoration: none;
}

/* —— Typography —— */
.eyebrow {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.78rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--cyan);
  margin: 0 0 8px;
}

h1 {
  font-size: clamp(1.85rem, 4vw, 2.6rem);
  font-weight: 700;
  letter-spacing: -0.02em;
  line-height: 1.15;
  margin: 0 0 12px;
}

h2 {
  font-size: clamp(1.25rem, 2.4vw, 1.55rem);
  font-weight: 650;
  margin: 0 0 10px;
  letter-spacing: -0.015em;
}

h3 {
  font-size: 1.05rem;
  font-weight: 600;
  margin: 0 0 8px;
}

.lead {
  color: var(--muted);
  font-size: 1.05rem;
  max-width: 62ch;
  margin: 0 0 28px;
}

.prose {
  color: var(--ink);
  max-width: 68ch;
}

.prose p {
  margin: 0 0 12px;
}

.prose ul {
  margin: 0 0 14px;
  padding-left: 1.2em;
  color: var(--muted);
}

.prose li {
  margin-bottom: 6px;
}

.prose li strong {
  color: var(--ink);
}

.formula {
  display: block;
  margin: 14px 0;
  padding: 12px 16px;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.92rem;
  color: var(--cyan);
  overflow-x: auto;
}

.hint {
  color: var(--dim);
  font-size: 0.9rem;
  margin: 8px 0 0;
}

/* —— Sections —— */
.section {
  padding: 36px 0 8px;
  scroll-margin-top: 20px;
}

.section + .section {
  border-top: 1px solid var(--line);
  margin-top: 28px;
}

.split {
  display: grid;
  grid-template-columns: 1fr 1.15fr;
  gap: 28px 36px;
  align-items: start;
}

@media (max-width: 860px) {
  .split {
    grid-template-columns: 1fr;
  }
}

/* —— Controls —— */
.controls {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 12px 0;
  align-items: center;
}

.btn {
  appearance: none;
  border: 1px solid var(--line);
  background: var(--bg-2);
  color: var(--ink);
  font-family: inherit;
  font-size: 0.88rem;
  font-weight: 500;
  padding: 7px 12px;
  border-radius: var(--radius);
  cursor: pointer;
  transition: border-color 0.15s, background 0.15s, color 0.15s;
}

.btn:hover {
  border-color: var(--cyan-dim);
  color: var(--cyan);
}

.btn:focus-visible {
  outline: 2px solid var(--cyan);
  outline-offset: 2px;
}

.btn.active,
.btn[aria-pressed="true"] {
  background: rgba(62, 199, 214, 0.14);
  border-color: var(--cyan);
  color: var(--cyan);
}

.btn.primary {
  background: rgba(62, 199, 214, 0.18);
  border-color: var(--cyan-dim);
  color: var(--cyan);
}

.btn.ghost {
  background: transparent;
}

select.group-select {
  appearance: none;
  background: var(--bg-2);
  color: var(--ink);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 7px 28px 7px 12px;
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.88rem;
  cursor: pointer;
  background-image: linear-gradient(45deg, transparent 50%, var(--muted) 50%),
    linear-gradient(135deg, var(--muted) 50%, transparent 50%);
  background-position: calc(100% - 14px) 50%, calc(100% - 9px) 50%;
  background-size: 5px 5px, 5px 5px;
  background-repeat: no-repeat;
}

select.group-select:focus-visible {
  outline: 2px solid var(--cyan);
  outline-offset: 2px;
}

.status {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.85rem;
  color: var(--muted);
  min-height: 1.4em;
}

.status .ok {
  color: var(--green);
}

.status .fail {
  color: var(--rose);
}

.status .accent {
  color: var(--cyan);
}

kbd {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.8em;
  padding: 1px 6px;
  border: 1px solid var(--line);
  border-radius: 3px;
  background: var(--bg-2);
  color: var(--muted);
}

/* —— Cayley table —— */
.cayley-wrap {
  overflow-x: auto;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
}

.cayley {
  border-collapse: collapse;
  width: 100%;
  min-width: max-content;
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.82rem;
}

.cayley th,
.cayley td {
  border: 1px solid var(--line);
  padding: 8px 10px;
  text-align: center;
  min-width: 2.6em;
  user-select: none;
}

.cayley th {
  background: rgba(62, 199, 214, 0.08);
  color: var(--muted);
  font-weight: 500;
}

.cayley th.corner {
  color: var(--dim);
}

.cayley td {
  cursor: pointer;
  color: var(--ink);
  transition: background 0.12s, color 0.12s;
}

.cayley td:hover {
  background: rgba(62, 199, 214, 0.1);
}

.cayley td.hl-row,
.cayley th.hl-row {
  background: rgba(62, 199, 214, 0.12);
}

.cayley td.hl-col,
.cayley th.hl-col {
  background: rgba(62, 199, 214, 0.12);
}

.cayley td.hl-cell {
  background: rgba(62, 199, 214, 0.28);
  color: var(--cyan);
  font-weight: 600;
}

.cayley td.hl-id {
  box-shadow: inset 0 0 0 1px var(--green);
  color: var(--green);
}

.cayley td.hl-inv {
  box-shadow: inset 0 0 0 1px var(--amber);
  color: var(--amber);
}

.cayley td.hl-assoc-a {
  background: rgba(62, 199, 214, 0.2);
}

.cayley td.hl-assoc-b {
  background: rgba(123, 201, 111, 0.2);
}

.cayley td.hl-assoc-c {
  background: rgba(224, 180, 90, 0.22);
}

.cayley td.hl-assoc-result {
  outline: 2px solid var(--cyan);
  outline-offset: -2px;
}

/* —— Viz panels —— */
.viz-panel {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  overflow: hidden;
  position: relative;
}

.viz-canvas {
  width: 100%;
  height: min(380px, 55vw);
  display: block;
  touch-action: none;
}

.viz-canvas canvas {
  display: block;
  width: 100% !important;
  height: 100% !important;
}

.viz-caption {
  padding: 10px 14px;
  border-top: 1px solid var(--line);
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.85rem;
  color: var(--muted);
}

/* —— Lesson index cards —— */
.lesson-list {
  list-style: none;
  padding: 0;
  margin: 24px 0 0;
  display: grid;
  gap: 12px;
}

.lesson-card {
  display: block;
  padding: 18px 20px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--panel);
  color: inherit;
  transition: border-color 0.15s, background 0.15s;
}

.lesson-card:hover {
  border-color: var(--cyan-dim);
  text-decoration: none;
  background: rgba(62, 199, 214, 0.06);
}

.lesson-card .num {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.78rem;
  color: var(--cyan);
  letter-spacing: 0.06em;
}

.lesson-card h2 {
  font-size: 1.2rem;
  margin: 4px 0 6px;
}

.lesson-card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

.toc {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 16px;
  margin: 0 0 8px;
  padding: 0;
  list-style: none;
  font-size: 0.9rem;
}

.toc a {
  color: var(--muted);
}

.toc a:hover {
  color: var(--cyan);
}

.axioms {
  display: grid;
  gap: 10px;
  margin: 16px 0;
}

.axiom {
  padding: 12px 14px;
  border-left: 2px solid var(--cyan-dim);
  background: rgba(10, 28, 40, 0.5);
}

.axiom .label {
  font-family: "JetBrains Mono", ui-monospace, monospace;
  font-size: 0.75rem;
  color: var(--cyan);
  letter-spacing: 0.06em;
  text-transform: uppercase;
  margin-bottom: 4px;
}

.axiom p {
  margin: 0;
  color: var(--muted);
  font-size: 0.95rem;
}

.site-footer {
  margin-top: 48px;
  padding: 20px 0 32px;
  border-top: 1px solid var(--line);
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
  gap: 12px;
  color: var(--dim);
  font-size: 0.85rem;
}

.next-box {
  margin-top: 20px;
  padding: 16px 18px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: rgba(62, 199, 214, 0.05);
}

.next-box h3 {
  color: var(--cyan);
}