:root {
  --bg: #f2efe8;
  --paper: #fbf9f4;
  --paper-strong: #ffffff;
  --ink: #1b1d22;
  --ink-soft: #59616d;
  --line: #d8d1c2;
  --line-strong: #b8b0a1;
  --accent: #0f6d7a;
  --accent-soft: rgba(15, 109, 122, 0.1);
  --accent-strong: #0b5261;
  --amber: #9a6125;
  --amber-soft: rgba(154, 97, 37, 0.1);
  --green: #2d6a4f;
  --green-soft: rgba(45, 106, 79, 0.1);
  --rose: #8c3b32;
  --rose-soft: rgba(140, 59, 50, 0.1);
  --shadow: 0 20px 60px rgba(30, 30, 30, 0.08);
  --body: "Source Serif 4", Georgia, serif;
  --heading: "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  --mono: "JetBrains Mono", monospace;
  --page: min(1380px, calc(100vw - 32px));
  --rail: 300px;
  --content: 860px;
  --font-heading: var(--heading);
  --bg-surface: var(--paper);
  --bg-elevated: var(--paper-strong);
  --border: var(--line);
  --text-primary: var(--ink);
  --text-secondary: var(--ink-soft);
  --text-muted: var(--ink-soft);
  --accent-dim: var(--accent-soft);
}

html[data-theme="dark"] {
  --bg: #11151b;
  --paper: #191f29;
  --paper-strong: #212937;
  --ink: #edf1f7;
  --ink-soft: #a9b2c2;
  --line: #313a4b;
  --line-strong: #556079;
  --accent: #6dc2cb;
  --accent-soft: rgba(109, 194, 203, 0.12);
  --accent-strong: #8fe1e8;
  --amber: #f0b56c;
  --amber-soft: rgba(240, 181, 108, 0.12);
  --green: #7bc99c;
  --green-soft: rgba(123, 201, 156, 0.12);
  --rose: #f08e84;
  --rose-soft: rgba(240, 142, 132, 0.12);
  --shadow: 0 24px 72px rgba(0, 0, 0, 0.35);
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background:
    radial-gradient(circle at top left, rgba(15, 109, 122, 0.08), transparent 28%),
    radial-gradient(circle at bottom right, rgba(154, 97, 37, 0.08), transparent 24%),
    var(--bg);
  color: var(--ink);
  font-family: var(--body);
  font-size: 18px;
  line-height: 1.7;
}

a {
  color: var(--accent);
  text-decoration-thickness: 1px;
  text-underline-offset: 0.15em;
}

a:hover {
  color: var(--accent-strong);
}

.page {
  width: var(--page);
  margin: 20px auto 40px;
  display: grid;
  grid-template-columns: var(--rail) minmax(0, 1fr);
  gap: 24px;
  align-items: start;
}

.rail {
  position: sticky;
  top: 18px;
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 18px;
  box-shadow: var(--shadow);
  padding: 20px;
}

.eyebrow {
  font-family: var(--heading);
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 0.16em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 12px;
}

.rail h1,
.rail h2,
.rail h3,
main h1,
main h2,
main h3 {
  font-family: var(--heading);
  line-height: 1.08;
}

.rail-title {
  font-size: 30px;
  margin: 0 0 10px;
}

.rail-copy,
.rail-note,
.rail-list {
  color: var(--ink-soft);
  font-size: 15px;
}

.rail-copy {
  margin: 0 0 16px;
}

.rail-card {
  margin-top: 14px;
  background: var(--paper-strong);
  border: 1px solid var(--line);
  border-radius: 14px;
  padding: 14px;
}

.rail-card h2,
.rail-card h3 {
  margin: 0 0 8px;
  font-size: 13px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--ink-soft);
}

.rail-card ul,
.rail-card ol {
  margin: 0;
  padding-left: 18px;
}

.rail-card li + li {
  margin-top: 8px;
}

.rail-nav {
  display: grid;
  gap: 8px;
}

.rail-nav a {
  display: block;
  text-decoration: none;
  font-family: var(--heading);
  font-size: 14px;
  color: var(--ink-soft);
  border: 1px solid transparent;
  border-radius: 10px;
  padding: 9px 10px;
  transition: transform 120ms ease, border-color 120ms ease, background 120ms ease, color 120ms ease;
}

.rail-nav a:hover,
.rail-nav a.active {
  color: var(--ink);
  border-color: var(--line);
  background: var(--accent-soft);
  transform: translateX(2px);
}

.rail-actions {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin-top: 16px;
}

.btn,
.rail-actions button,
.rail-actions a {
  appearance: none;
  border: 1px solid var(--line);
  background: transparent;
  color: var(--ink);
  font-family: var(--heading);
  font-size: 13px;
  font-weight: 700;
  border-radius: 10px;
  padding: 10px 12px;
  cursor: pointer;
  text-decoration: none;
}

.btn:hover,
.rail-actions button:hover,
.rail-actions a:hover {
  border-color: var(--accent);
  background: var(--accent-soft);
  color: var(--accent-strong);
}

main {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: 18px;
  box-shadow: var(--shadow);
  overflow: hidden;
}

.hero {
  padding: 38px 44px 30px;
  background:
    linear-gradient(180deg, rgba(15, 109, 122, 0.08), transparent 58%),
    var(--paper);
  border-bottom: 1px solid var(--line);
}

.hero h1 {
  margin: 0 0 12px;
  font-size: clamp(38px, 4.6vw, 58px);
  letter-spacing: -0.035em;
}

.subtitle {
  margin: 0 0 16px;
  max-width: 48rem;
  color: var(--ink-soft);
  font-family: var(--heading);
  font-size: 17px;
}

.hero-lead {
  margin: 0;
  max-width: 44rem;
  font-size: 23px;
  line-height: 1.55;
}

.hero-pills {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin-top: 18px;
}

.pill {
  display: inline-flex;
  align-items: center;
  border: 1px solid var(--line);
  background: var(--paper-strong);
  border-radius: 999px;
  padding: 7px 12px;
  font-family: var(--heading);
  font-size: 12px;
  font-weight: 700;
  color: var(--ink-soft);
}

.content {
  padding: 34px 44px 56px;
}

section {
  padding-top: 8px;
}

section + section {
  margin-top: 42px;
}

section h2 {
  margin: 0 0 14px;
  padding-top: 22px;
  border-top: 1px solid var(--line);
  font-size: 29px;
  letter-spacing: -0.02em;
}

section h3 {
  margin: 28px 0 12px;
  font-size: 20px;
}

section p,
section li {
  max-width: 43rem;
}

p {
  margin: 0 0 18px;
}

ul,
ol {
  margin: 0 0 18px;
  padding-left: 22px;
}

li + li {
  margin-top: 8px;
}

.section-kicker {
  margin: 0 0 10px;
  font-family: var(--heading);
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--accent);
}

.section-intro {
  margin: 0 0 18px;
  max-width: 44rem;
  color: var(--ink-soft);
  font-size: 19px;
}

.callout,
.commentary,
.equation-box,
.worked-example,
.figure-block,
.table-card {
  max-width: 48rem;
  margin: 20px 0 24px;
  border: 1px solid var(--line);
  border-radius: 16px;
  padding: 18px 18px 16px;
  background: var(--paper-strong);
}

.callout {
  background: linear-gradient(135deg, var(--accent-soft), transparent 55%), var(--paper-strong);
}

.commentary {
  background: linear-gradient(135deg, var(--amber-soft), transparent 58%), var(--paper-strong);
}

.equation-box {
  background: linear-gradient(135deg, var(--green-soft), transparent 58%), var(--paper-strong);
}

.worked-example {
  background: linear-gradient(135deg, var(--rose-soft), transparent 58%), var(--paper-strong);
}

.box-label {
  margin: 0 0 10px;
  font-family: var(--heading);
  font-size: 12px;
  font-weight: 800;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--ink-soft);
}

.equation {
  margin: 10px 0;
  padding: 14px 16px;
  border-radius: 12px;
  background: rgba(255, 255, 255, 0.45);
  border: 1px solid var(--line);
  font-family: var(--mono);
  font-size: 14px;
  line-height: 1.65;
  white-space: pre-wrap;
}

html[data-theme="dark"] .equation {
  background: rgba(0, 0, 0, 0.12);
}

.figure-block svg {
  width: 100%;
  height: auto;
  display: block;
  border-radius: 12px;
}

.fig-caption {
  margin-top: 12px;
  color: var(--ink-soft);
  font-size: 14px;
}

.figure-title {
  margin: 0 0 12px;
  font-family: var(--heading);
  font-size: 14px;
  font-weight: 700;
  color: var(--ink);
}

.table-card table {
  width: 100%;
  border-collapse: collapse;
  font-size: 15px;
}

.table-card th,
.table-card td {
  border-bottom: 1px solid var(--line);
  text-align: left;
  padding: 10px 8px;
  vertical-align: top;
}

.table-card th {
  font-family: var(--heading);
  font-size: 12px;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--ink-soft);
}

.table-card tr:last-child td {
  border-bottom: none;
}

.term {
  color: var(--accent);
  font-weight: 700;
}

.small {
  color: var(--ink-soft);
  font-size: 15px;
}

.summary-grid {
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  max-width: 52rem;
  margin-top: 22px;
}

.summary-card {
  border: 1px solid var(--line);
  border-radius: 16px;
  background: var(--paper-strong);
  padding: 18px;
}

.summary-card h3 {
  margin: 0 0 8px;
  font-size: 18px;
}

.footer {
  margin-top: 40px;
  padding-top: 20px;
  border-top: 1px solid var(--line);
  color: var(--ink-soft);
  font-size: 15px;
}

.hub {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
  margin-top: 20px;
}

.hub-card {
  display: block;
  text-decoration: none;
  color: inherit;
  border: 1px solid var(--line);
  border-radius: 18px;
  background: var(--paper-strong);
  padding: 22px;
  transition: transform 120ms ease, border-color 120ms ease, background 120ms ease;
}

.hub-card:hover {
  transform: translateY(-2px);
  border-color: var(--accent);
  background: var(--accent-soft);
}

.hub-card h2 {
  border-top: none;
  padding-top: 0;
  margin-bottom: 10px;
  font-size: 24px;
}

.hub-card p,
.hub-card li {
  color: var(--ink-soft);
}

.hub-card ul {
  margin-top: 10px;
}

.badge {
  display: inline-block;
  margin-bottom: 12px;
  border-radius: 999px;
  padding: 5px 10px;
  font-family: var(--heading);
  font-size: 11px;
  font-weight: 800;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  border: 1px solid var(--line);
  color: var(--ink-soft);
}

@media (max-width: 1180px) {
  .page {
    grid-template-columns: 1fr;
  }

  .rail {
    position: static;
  }
}

@media (max-width: 800px) {
  body {
    font-size: 17px;
  }

  .page {
    width: min(100vw - 18px, 100%);
    margin: 9px auto 20px;
    gap: 12px;
  }

  .hero,
  .content,
  .rail {
    padding-left: 18px;
    padding-right: 18px;
  }

  .summary-grid,
  .hub {
    grid-template-columns: 1fr;
  }

  .hero h1 {
    font-size: 34px;
  }

  .hero-lead {
    font-size: 20px;
  }
}