:root {
  color-scheme: light;
  --bg: #f7f4ee;
  --surface: #fffdf8;
  --surface-2: #f0ebe1;
  --ink: #24211d;
  --muted: #6c655c;
  --line: #d8d0c2;
  --accent: #116a78;
  --accent-2: #8f3f25;
  --code-bg: #1f2528;
  --code-ink: #eef7f6;
  --mark: #fff1a8;
  --shadow: 0 18px 45px rgba(51, 42, 30, 0.12);
  --radius: 8px;
  --sidebar: 320px;
}

:root.dark {
  color-scheme: dark;
  --bg: #15171a;
  --surface: #1f2327;
  --surface-2: #292e32;
  --ink: #f1ece2;
  --muted: #bcb3a8;
  --line: #3d444a;
  --accent: #78c7d3;
  --accent-2: #f2a477;
  --code-bg: #0d1012;
  --code-ink: #e8f2ef;
  --mark: #665a18;
  --shadow: 0 18px 45px rgba(0, 0, 0, 0.32);
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: -apple-system, BlinkMacSystemFont, "Apple SD Gothic Neo", "Noto Sans KR", "Segoe UI", sans-serif;
  line-height: 1.72;
  text-rendering: optimizeLegibility;
}

a {
  color: inherit;
}

button,
input {
  font: inherit;
}

.progress {
  position: fixed;
  inset: 0 0 auto;
  z-index: 40;
  height: 4px;
  background: transparent;
}

.progress__bar {
  width: 0;
  height: 100%;
  background: linear-gradient(90deg, var(--accent), var(--accent-2));
  transition: width 120ms linear;
}

.layout {
  display: grid;
  grid-template-columns: var(--sidebar) minmax(0, 1fr);
  min-height: 100vh;
}

.sidebar {
  position: sticky;
  top: 0;
  height: 100vh;
  overflow: auto;
  border-right: 1px solid var(--line);
  background: color-mix(in srgb, var(--surface) 92%, var(--bg));
  padding: 28px 20px;
}

.brand {
  display: grid;
  gap: 10px;
  margin-bottom: 18px;
}

.brand__eyebrow {
  color: var(--accent-2);
  font-size: 0.78rem;
  font-weight: 750;
  letter-spacing: 0;
}

.brand h1 {
  margin: 0;
  font-size: 1.45rem;
  line-height: 1.22;
}

.brand p {
  margin: 0;
  color: var(--muted);
  font-size: 0.93rem;
}

.toolbar {
  display: grid;
  gap: 10px;
  margin: 20px 0;
}

.search {
  position: relative;
}

.search input {
  width: 100%;
  min-height: 42px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--ink);
  padding: 9px 12px;
  outline: none;
}

.search input:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 24%, transparent);
}

.search-count {
  min-height: 20px;
  color: var(--muted);
  font-size: 0.82rem;
}

.controls {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 8px;
}

.icon-btn,
.nav-btn,
.collapse-btn {
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  color: var(--ink);
  cursor: pointer;
}

.icon-btn {
  min-height: 38px;
  padding: 7px 10px;
}

.icon-btn:hover,
.nav-btn:hover,
.collapse-btn:hover {
  border-color: color-mix(in srgb, var(--accent) 72%, var(--line));
  color: var(--accent);
}

.toc-title {
  margin: 18px 0 8px;
  color: var(--muted);
  font-size: 0.8rem;
  font-weight: 800;
}

.toc {
  display: grid;
  gap: 2px;
  padding-bottom: 80px;
}

.toc a {
  display: block;
  border-radius: 6px;
  color: var(--muted);
  text-decoration: none;
  line-height: 1.3;
}

.toc a:hover {
  color: var(--accent);
  background: color-mix(in srgb, var(--accent) 10%, transparent);
}

.toc .toc-chapter {
  margin-top: 8px;
  padding: 9px 10px;
  color: var(--ink);
  font-weight: 800;
}

.toc .toc-section {
  padding: 6px 10px 6px 24px;
  font-size: 0.88rem;
}

.toc a.is-active {
  background: color-mix(in srgb, var(--accent) 16%, transparent);
  color: var(--accent);
}

.content {
  min-width: 0;
  padding: 44px clamp(20px, 5vw, 70px) 120px;
}

.book {
  width: min(100%, 980px);
  margin: 0 auto;
}

.hero {
  margin-bottom: 34px;
  padding-bottom: 30px;
  border-bottom: 1px solid var(--line);
}

.hero h2 {
  margin: 0 0 12px;
  font-size: clamp(2.2rem, 5vw, 4.8rem);
  line-height: 1.04;
  letter-spacing: 0;
}

.hero p {
  max-width: 760px;
  margin: 0 0 12px;
  color: var(--muted);
  font-size: 1.05rem;
}

.note {
  border-left: 4px solid var(--accent);
  background: color-mix(in srgb, var(--accent) 9%, var(--surface));
  padding: 14px 16px;
  border-radius: 0 var(--radius) var(--radius) 0;
}

.chapter {
  margin: 42px 0;
  padding: 30px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface);
  box-shadow: var(--shadow);
  scroll-margin-top: 20px;
}

.chapter > h2,
.chapter-header h2 {
  display: flex;
  align-items: start;
  justify-content: space-between;
  gap: 16px;
  margin: 0 0 22px;
  font-size: clamp(1.7rem, 2.4vw, 2.4rem);
  line-height: 1.18;
}

.chapter-header {
  margin-bottom: 22px;
}

.chapter-kicker {
  margin: 0 0 6px;
  color: var(--accent-2);
  font-size: 0.84rem;
  font-weight: 850;
}

.chapter-header p:last-child {
  margin-bottom: 0;
}

.chapter-number {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-width: 2.2rem;
  height: 2.2rem;
  margin-right: 10px;
  border-radius: 999px;
  background: var(--ink);
  color: var(--surface);
  font-size: 1.05rem;
  vertical-align: 0.16em;
}

.book-section {
  margin: 24px 0 0;
  padding-top: 20px;
  border-top: 1px solid var(--line);
  scroll-margin-top: 22px;
}

.book-section > h3 {
  display: flex;
  align-items: start;
  justify-content: space-between;
  gap: 16px;
  margin: 0 0 12px;
  font-size: clamp(1.2rem, 1.8vw, 1.55rem);
  line-height: 1.28;
}

.book-section h4 {
  margin: 22px 0 8px;
  font-size: 1rem;
  color: var(--accent-2);
}

.book-section p,
.book-section ul,
.book-section ol {
  margin: 12px 0;
}

.book-section li + li {
  margin-top: 6px;
}

.collapse-btn {
  flex: 0 0 auto;
  min-width: 34px;
  height: 34px;
  padding: 0;
  line-height: 1;
  font-weight: 800;
}

.is-collapsed > .section-body,
.is-collapsed > .chapter-body {
  display: none;
}

.formula {
  overflow-x: auto;
  margin: 16px 0;
  padding: 12px 14px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: var(--surface-2);
  font-family: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
  white-space: pre-wrap;
}

pre {
  overflow-x: auto;
  margin: 16px 0;
  border-radius: var(--radius);
  background: var(--code-bg);
  color: var(--code-ink);
}

code {
  font-family: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;
  font-size: 0.92em;
}

pre code {
  display: block;
  padding: 16px;
  line-height: 1.55;
}

.term {
  font-weight: 800;
  color: var(--accent);
}

.example,
.proof,
.warning {
  margin: 16px 0;
  padding: 14px 16px;
  border: 1px solid var(--line);
  border-radius: var(--radius);
  background: color-mix(in srgb, var(--surface-2) 65%, var(--surface));
}

.proof strong,
.example strong,
.warning strong {
  color: var(--accent-2);
}

.exercise-list {
  padding-left: 1.2rem;
}

mark {
  background: var(--mark);
  color: inherit;
  border-radius: 3px;
  padding: 0 2px;
}

.search-hidden {
  display: none;
}

.reader-nav {
  position: fixed;
  right: clamp(16px, 3vw, 34px);
  bottom: 22px;
  z-index: 30;
  display: flex;
  gap: 8px;
}

.nav-btn {
  min-height: 42px;
  padding: 8px 14px;
  box-shadow: var(--shadow);
}

.footer {
  width: min(100%, 980px);
  margin: 50px auto 0;
  padding-top: 24px;
  border-top: 1px solid var(--line);
  color: var(--muted);
  font-size: 0.94rem;
}

.mobile-top {
  display: none;
}

@media (max-width: 920px) {
  .layout {
    display: block;
  }

  .mobile-top {
    position: sticky;
    top: 4px;
    z-index: 35;
    display: flex;
    justify-content: space-between;
    gap: 8px;
    border-bottom: 1px solid var(--line);
    background: var(--surface);
    padding: 10px 14px;
  }

  .sidebar {
    position: fixed;
    inset: 4px 0 0;
    z-index: 34;
    width: min(88vw, 360px);
    transform: translateX(-105%);
    transition: transform 180ms ease;
    box-shadow: var(--shadow);
  }

  body.toc-open .sidebar {
    transform: translateX(0);
  }

  .content {
    padding: 24px 16px 110px;
  }

  .chapter {
    padding: 22px 16px;
  }

  .reader-nav {
    right: 12px;
    left: 12px;
  }

  .nav-btn {
    flex: 1;
  }
}

@media (prefers-reduced-motion: reduce) {
  html {
    scroll-behavior: auto;
  }

  * {
    transition: none !important;
  }
}