:root {
  --bg: #fbfaf8;
  --bg-alt: #f2efe9;
  --text: #1f2320;
  --text-muted: #5a5f5a;
  --accent: #7a4b2a;
  --accent-strong: #5c3a20;
  --border: #ddd6c9;
  --card-bg: #ffffff;
  --code-bg: #f0ede4;
  --link: #6a3d1f;
  --link-hover: #4a2a12;
  --sidebar-bg: #f5f2ea;
  --shadow: rgba(31, 35, 32, 0.06);
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #16181a;
    --bg-alt: #1d2023;
    --text: #e7e4dc;
    --text-muted: #a5a49b;
    --accent: #d8a978;
    --accent-strong: #e9c199;
    --border: #33362f;
    --card-bg: #1e2124;
    --code-bg: #24272a;
    --link: #d8a978;
    --link-hover: #f0c497;
    --sidebar-bg: #1a1c1e;
    --shadow: rgba(0, 0, 0, 0.35);
  }
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, "Times New Roman", serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.65;
  font-size: 17px;
}

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

/* Sidebar */
.sidebar {
  width: 250px;
  flex-shrink: 0;
  background: var(--sidebar-bg);
  border-right: 1px solid var(--border);
  padding: 1.75rem 1.25rem;
  position: sticky;
  top: 0;
  height: 100vh;
  overflow-y: auto;
}

.site-title {
  font-size: 1.05rem;
  font-weight: 700;
  letter-spacing: 0.02em;
  color: var(--text);
  text-decoration: none;
  display: block;
  margin-bottom: 0.15rem;
}

.site-subtitle {
  font-size: 0.75rem;
  color: var(--text-muted);
  margin-bottom: 1.5rem;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  letter-spacing: 0.03em;
  text-transform: uppercase;
}

.nav-group-label {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-muted);
  margin: 1.2rem 0 0.4rem;
}

nav.toc ul {
  list-style: none;
  margin: 0;
  padding: 0;
}

nav.toc li {
  margin: 0.05rem 0;
}

nav.toc a {
  display: block;
  padding: 0.35rem 0.6rem;
  border-radius: 6px;
  color: var(--text);
  text-decoration: none;
  font-size: 0.92rem;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  border-left: 2px solid transparent;
}

nav.toc a:hover {
  background: var(--bg-alt);
  color: var(--link);
}

nav.toc a.active {
  color: var(--accent-strong);
  border-left: 2px solid var(--accent);
  background: var(--bg-alt);
  font-weight: 600;
}

.nav-index {
  font-variant-numeric: tabular-nums;
  color: var(--text-muted);
  margin-right: 0.4rem;
}

/* Main content */
main {
  flex: 1;
  min-width: 0;
  padding: 3rem 3.5rem 5rem;
  max-width: 860px;
}

.eyebrow {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  font-size: 0.78rem;
  letter-spacing: 0.1em;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 0.5rem;
  font-weight: 600;
}

h1 {
  font-size: 2.1rem;
  margin: 0 0 1rem;
  line-height: 1.25;
}

h2 {
  font-size: 1.45rem;
  margin: 2.4rem 0 0.9rem;
  padding-bottom: 0.4rem;
  border-bottom: 1px solid var(--border);
}

h3 {
  font-size: 1.15rem;
  margin: 1.8rem 0 0.6rem;
  color: var(--accent-strong);
}

p {
  margin: 0 0 1rem;
}

.lede {
  font-size: 1.15rem;
  color: var(--text-muted);
  margin-bottom: 1.75rem;
}

.hero-figure {
  margin: 0 0 1.75rem;
}

.hero-figure img {
  display: block;
  width: 100%;
  height: auto;
  border-radius: 12px;
  border: 1px solid var(--border);
  box-shadow: 0 2px 10px var(--shadow);
}

.hero-figure figcaption {
  margin-top: 0.6rem;
  font-size: 0.85rem;
  color: var(--text-muted);
  text-align: center;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

a {
  color: var(--link);
}
a:hover {
  color: var(--link-hover);
}

code, .katex {
  font-size: 1em;
}

code {
  background: var(--code-bg);
  padding: 0.12rem 0.35rem;
  border-radius: 4px;
  font-family: "SF Mono", Menlo, Consolas, monospace;
  font-size: 0.85em;
}

.box {
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 1.1rem 1.4rem;
  margin: 1.4rem 0;
  box-shadow: 0 1px 3px var(--shadow);
}

.box > :last-child {
  margin-bottom: 0;
}

.box-label {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  font-size: 0.72rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  font-weight: 700;
  color: var(--accent);
  margin-bottom: 0.5rem;
  display: block;
}

.box.definition { border-left: 3px solid #4a7a6a; }
.box.definition .box-label { color: #4a7a6a; }
.box.theorem { border-left: 3px solid var(--accent); }
.box.example { border-left: 3px solid #6a6f9a; }
.box.example .box-label { color: #6a6f9a; }
.box.remark { border-left: 3px solid #a5a49b; }
.box.remark .box-label { color: var(--text-muted); }

ul, ol {
  padding-left: 1.4rem;
}

li {
  margin-bottom: 0.4rem;
}

.katex-display {
  overflow-x: auto;
  overflow-y: hidden;
  padding: 0.2rem 0;
}

.page-nav {
  display: flex;
  justify-content: space-between;
  margin-top: 3.5rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  gap: 1rem;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

.page-nav a {
  text-decoration: none;
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 0.7rem 1rem;
  flex: 1;
  max-width: 48%;
}

.page-nav a:hover {
  border-color: var(--accent);
}

.page-nav .dir {
  display: block;
  font-size: 0.7rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: var(--text-muted);
  margin-bottom: 0.2rem;
}

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

.menu-toggle {
  display: none;
}

.home-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(240px, 1fr));
  gap: 1rem;
  margin-top: 1.5rem;
}

.home-card {
  display: block;
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 1.1rem 1.3rem;
  text-decoration: none;
  color: var(--text);
  box-shadow: 0 1px 3px var(--shadow);
  transition: transform 0.12s ease, border-color 0.12s ease;
}

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

.home-card .num {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  font-size: 0.75rem;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: 0.05em;
}

.home-card h3 {
  margin: 0.3rem 0 0.35rem;
  color: var(--text);
}

.home-card p {
  margin: 0;
  font-size: 0.9rem;
  color: var(--text-muted);
}

footer.site-footer {
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
  font-size: 0.82rem;
  color: var(--text-muted);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
}

@media (max-width: 860px) {
  .layout {
    flex-direction: column;
  }
  .sidebar {
    position: relative;
    width: 100%;
    height: auto;
    border-right: none;
    border-bottom: 1px solid var(--border);
  }
  main {
    padding: 2rem 1.4rem 4rem;
  }
}