:root {
  --ink: #172033;
  --muted: #5f6d7a;
  --line: #d9e2ec;
  --wash: #f5f8fb;
  --panel: #ffffff;
  --soft: #eef4fb;
  --teal: #0f766e;
  --blue: #1d4ed8;
  --violet: #6d28d9;
  --amber: #b45309;
  --code: #101827;
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  color: var(--ink);
  background: var(--wash);
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  line-height: 1.62;
}

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

a:hover {
  text-decoration: underline;
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 20;
  border-bottom: 1px solid var(--line);
  background: rgba(255, 255, 255, .94);
  backdrop-filter: blur(12px);
}

.site-header-inner {
  width: min(1180px, calc(100% - 32px));
  min-height: 64px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
}

.brand {
  color: var(--ink);
  font-weight: 800;
  letter-spacing: 0;
}

.top-links {
  display: flex;
  align-items: center;
  flex-wrap: wrap;
  gap: 12px;
  font-size: 14px;
  font-weight: 650;
}

.page-shell {
  width: min(1180px, calc(100% - 32px));
  margin: 0 auto;
  padding: 32px 0 56px;
  display: grid;
  grid-template-columns: 260px minmax(0, 1fr);
  gap: 28px;
}

.sidebar {
  align-self: start;
  position: sticky;
  top: 88px;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel);
  padding: 18px;
}

.sidebar-title {
  margin: 0 0 10px;
  color: var(--muted);
  font-size: 12px;
  font-weight: 800;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.sidebar nav {
  display: grid;
  gap: 6px;
}

.sidebar a {
  display: block;
  border-radius: 6px;
  padding: 7px 8px;
  color: #28364a;
  font-size: 14px;
  font-weight: 600;
}

.sidebar a:hover {
  background: var(--soft);
  text-decoration: none;
}

.lesson {
  min-width: 0;
  border: 1px solid var(--line);
  border-radius: 8px;
  background: var(--panel);
  padding: 36px;
  box-shadow: 0 18px 38px rgba(23, 32, 51, .06);
}

.source-note {
  margin: 0 0 24px;
  border-left: 4px solid var(--teal);
  border-radius: 6px;
  background: #edfdfa;
  padding: 12px 14px;
  color: #134e4a;
  font-size: 14px;
}

.lesson h1 {
  margin: 0 0 18px;
  color: #111827;
  font-size: clamp(34px, 5vw, 52px);
  line-height: 1.05;
  letter-spacing: 0;
}

.lesson h2 {
  margin: 38px 0 12px;
  padding-top: 10px;
  border-top: 1px solid var(--line);
  color: #172033;
  font-size: 26px;
  line-height: 1.2;
  letter-spacing: 0;
}

.lesson h3 {
  margin: 28px 0 10px;
  color: #233047;
  font-size: 20px;
  line-height: 1.25;
}

.lesson h4 {
  margin: 22px 0 8px;
  color: #2c3b52;
  font-size: 17px;
}

.lesson p,
.lesson li {
  font-size: 16px;
}

.lesson p {
  margin: 0 0 16px;
}

.lesson ul,
.lesson ol {
  margin: 0 0 18px 22px;
  padding: 0;
}

.lesson li + li {
  margin-top: 5px;
}

.lesson strong {
  color: #111827;
}

.lesson table {
  width: 100%;
  margin: 22px 0;
  border: 1px solid var(--line);
  border-collapse: collapse;
  border-radius: 8px;
  overflow: hidden;
  display: block;
  overflow-x: auto;
}

.lesson th,
.lesson td {
  padding: 11px 12px;
  border-bottom: 1px solid var(--line);
  border-right: 1px solid var(--line);
  text-align: left;
  vertical-align: top;
  min-width: 130px;
}

.lesson th {
  background: var(--soft);
  color: #1f2a44;
}

.lesson tr:last-child td {
  border-bottom: 0;
}

.lesson blockquote {
  margin: 22px 0;
  border-left: 4px solid var(--violet);
  border-radius: 6px;
  background: #f6f0ff;
  padding: 14px 16px;
  color: #33204f;
}

.lesson code {
  border-radius: 5px;
  background: #eef2f7;
  padding: 2px 5px;
  color: #172033;
  font-size: .92em;
}

.lesson pre {
  overflow-x: auto;
  border-radius: 8px;
  background: var(--code);
  padding: 16px;
}

.lesson pre code {
  background: transparent;
  padding: 0;
  color: #eef6ff;
}

.lesson img {
  max-width: 100%;
  border-radius: 8px;
  border: 1px solid var(--line);
}

.page-footer {
  border-top: 1px solid var(--line);
  padding: 26px 16px;
  color: var(--muted);
  text-align: center;
  font-size: 14px;
}

@media (max-width: 900px) {
  .site-header-inner {
    align-items: flex-start;
    flex-direction: column;
    padding: 14px 0;
  }

  .page-shell {
    grid-template-columns: 1fr;
  }

  .sidebar {
    position: static;
  }

  .lesson {
    padding: 24px;
  }
}

@media (max-width: 540px) {
  .page-shell,
  .site-header-inner {
    width: min(100% - 20px, 1180px);
  }

  .lesson {
    padding: 18px;
  }
}