*,
*::before,
*::after {
  box-sizing: border-box;
}

:root {
  --ink: #172033;
  --muted: #4d5b73;
  --soft: #6b7a91;
  --line: rgba(23, 32, 51, 0.12);
  --accent: #246b66;
  --accent-soft: rgba(36, 107, 102, 0.1);
  --panel: rgba(255, 255, 255, 0.78);
  --bg:
    radial-gradient(circle at top left, #d7e8f5 0%, transparent 42%),
    linear-gradient(165deg, #f4f1ea 0%, #e8eef4 52%, #dfe8e1 100%);
  --shell: min(66rem, calc(100% - 2rem));
  --sidebar: 14.75rem;
  --reading: 47rem;
  --font-ui: "Source Sans 3", "Segoe UI", sans-serif;
  --font-body: "Source Serif 4", Georgia, "Times New Roman", serif;
  --font-mono: "Cascadia Code", "Consolas", "Courier New", monospace;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  font-family: var(--font-body);
  color: var(--ink);
  background: var(--bg);
  line-height: 1.65;
}

.skip-link {
  position: absolute;
  left: -999px;
  top: 0;
  background: var(--ink);
  color: #fff;
  padding: 0.75rem 1rem;
  z-index: 1000;
  font-family: var(--font-ui);
}

.skip-link:focus {
  left: 0.5rem;
  top: 0.5rem;
}

.site-header {
  border-bottom: 1px solid var(--line);
  background: rgba(255, 255, 255, 0.82);
  backdrop-filter: blur(10px);
  position: sticky;
  top: 0;
  z-index: 50;
}

.header-inner,
.footer-inner,
.layout-shell {
  width: var(--shell);
  margin-left: auto;
  margin-right: auto;
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  min-height: 3.5rem;
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 0.85rem;
  font-family: var(--font-ui);
}

.brand {
  font-family: var(--font-ui);
  font-size: 1.05rem;
  font-weight: 700;
  color: var(--ink);
  text-decoration: none;
  letter-spacing: -0.02em;
}

.brand:hover,
.brand:focus-visible {
  color: var(--accent);
}

.site-nav {
  display: flex;
  gap: 0.25rem 1rem;
}

.site-nav a {
  color: var(--muted);
  text-decoration: none;
  font-size: 0.95rem;
  font-weight: 600;
}

.site-nav a:hover,
.site-nav a:focus-visible {
  color: var(--accent);
  text-decoration: underline;
}

.sidebar-toggle {
  display: none;
  appearance: none;
  border: 1px solid var(--line);
  background: #fff;
  color: var(--ink);
  padding: 0.4rem 0.8rem;
  font: inherit;
  font-family: var(--font-ui);
  font-size: 0.9rem;
  font-weight: 600;
  cursor: pointer;
}

.sidebar-toggle:hover,
.sidebar-toggle:focus-visible {
  border-color: var(--accent);
  color: var(--accent);
  outline: none;
}

.layout-shell {
  display: grid;
  grid-template-columns: var(--sidebar) minmax(0, 1fr);
  gap: 0 2.5rem;
  align-items: start;
  flex: 1;
  padding: 1.75rem 0 3.5rem;
}

.course-sidebar {
  position: sticky;
  top: 4.25rem;
  align-self: start;
  padding: 1.15rem 1rem 1.25rem;
  background: var(--panel);
  border: 1px solid var(--line);
  border-radius: 0.35rem;
  max-height: calc(100vh - 5.25rem);
  overflow: auto;
  font-family: var(--font-ui);
}

.sidebar-title {
  margin: 0 0 1rem;
  font-size: 0.72rem;
  letter-spacing: 0.12em;
  text-transform: uppercase;
  color: var(--soft);
  font-weight: 700;
}

.sidebar-group + .sidebar-group {
  margin-top: 0.95rem;
  padding-top: 0.95rem;
  border-top: 1px solid var(--line);
}

.sidebar-heading {
  margin: 0 0 0.35rem;
  padding-left: 0.55rem;
  font-size: 0.75rem;
  letter-spacing: 0.04em;
  text-transform: uppercase;
  color: var(--soft);
  font-weight: 700;
}

.sidebar-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: grid;
  gap: 0.1rem;
}

.sidebar-list a {
  display: block;
  padding: 0.42rem 0.55rem;
  color: var(--muted);
  text-decoration: none;
  border-radius: 0.25rem;
  border-left: 3px solid transparent;
  font-size: 0.95rem;
  font-weight: 600;
  line-height: 1.3;
}

.sidebar-list a:hover,
.sidebar-list a:focus-visible {
  color: var(--accent);
  background: var(--accent-soft);
  outline: none;
}

.sidebar-list a.is-active,
.sidebar-list a[aria-current="page"] {
  color: var(--ink);
  background: var(--accent-soft);
  border-left-color: var(--accent);
}

main {
  min-width: 0;
  max-width: var(--reading);
  padding-top: 0.15rem;
}

h1 {
  margin: 0 0 0.7rem;
  font-size: clamp(1.85rem, 4vw, 2.45rem);
  letter-spacing: -0.03em;
  line-height: 1.15;
  color: var(--ink);
}

h2 {
  margin: 1.85rem 0 0.7rem;
  font-size: 1.28rem;
  letter-spacing: -0.015em;
  color: var(--ink);
}

.lead {
  margin-bottom: 1.15rem;
  font-size: 1.18rem;
  color: var(--muted);
  font-family: var(--font-ui);
  font-weight: 400;
  line-height: 1.5;
}

p {
  margin: 0 0 1rem;
  font-size: 1.05rem;
  color: var(--muted);
}

.tutorial p,
.tutorial li {
  color: #2a3548;
}

.byline {
  margin-top: -0.15rem;
  margin-bottom: 1.4rem;
  font-family: var(--font-ui);
  font-size: 0.92rem;
  color: var(--soft);
}

.breadcrumbs {
  margin-bottom: 1rem;
  font-family: var(--font-ui);
  font-size: 0.9rem;
  color: var(--soft);
}

.breadcrumbs a {
  color: var(--accent);
  text-decoration: none;
  font-weight: 600;
}

.breadcrumbs a:hover,
.breadcrumbs a:focus-visible {
  text-decoration: underline;
}

ul.topic-list,
main > ul {
  list-style: none;
  margin: 0 0 1.5rem;
  padding: 0;
  display: grid;
  gap: 0.65rem;
}

ul.topic-list li a,
main > ul li a {
  display: block;
  padding: 0.95rem 1.15rem;
  text-decoration: none;
  color: var(--ink);
  background: var(--panel);
  border: 1px solid var(--line);
  border-left: 4px solid var(--accent);
  border-radius: 0 0.3rem 0.3rem 0;
  transition: background 0.15s ease, border-color 0.15s ease;
  font-family: var(--font-ui);
}

ul.topic-list li a:hover,
ul.topic-list li a:focus-visible,
main > ul li a:hover,
main > ul li a:focus-visible {
  background: #fff;
  border-color: rgba(36, 107, 102, 0.35);
  outline: none;
}

ul.topic-list li a strong,
main > ul li a strong {
  font-size: 1.05rem;
}

.topic-desc {
  display: block;
  margin-top: 0.3rem;
  font-weight: 400;
  color: var(--muted);
  font-size: 0.95rem;
}

.lesson-group {
  margin: 1.75rem 0 1.1rem;
}

.lesson-group h2 {
  margin: 0 0 0.75rem;
  font-size: 1.2rem;
}

ol {
  margin: 0 0 1.5rem;
  padding-left: 1.25rem;
  color: var(--ink);
}

ol li {
  margin-bottom: 0.5rem;
  color: var(--muted);
}

ul.plain {
  display: block;
  list-style: disc;
  margin: 0 0 1.5rem;
  padding-left: 1.25rem;
}

ul.plain li {
  margin-bottom: 0.45rem;
}

ul.plain a {
  color: var(--accent);
  font-weight: 600;
}

.related {
  margin-top: 2.25rem;
  padding-top: 1.35rem;
  border-top: 1px solid var(--line);
}

code,
kbd {
  font-family: var(--font-mono);
  font-size: 0.92em;
}

code {
  padding: 0.1em 0.35em;
  background: rgba(23, 32, 51, 0.07);
  border-radius: 0.2rem;
}

kbd {
  padding: 0.1em 0.4em;
  border: 1px solid rgba(23, 32, 51, 0.18);
  border-bottom-width: 2px;
  background: rgba(255, 255, 255, 0.8);
  border-radius: 0.2rem;
  font-family: var(--font-ui);
  font-size: 0.85em;
}

pre {
  margin: 0.85rem 0 1.25rem;
  padding: 1rem 1.15rem;
  overflow-x: auto;
  background: #172033;
  color: #e8eef4;
  border-left: 4px solid var(--accent);
  border-radius: 0 0.3rem 0.3rem 0;
}

pre code {
  padding: 0;
  background: none;
  color: inherit;
  font-size: 0.92rem;
  line-height: 1.55;
}

.note {
  padding: 0.85rem 1rem;
  background: rgba(180, 90, 40, 0.1);
  border-left: 4px solid #b45a28;
  color: #5a3a22;
  border-radius: 0 0.3rem 0.3rem 0;
}

.article-header {
  margin-bottom: 0.35rem;
}

.site-footer {
  margin-top: auto;
  border-top: 1px solid rgba(23, 32, 51, 0.2);
  background: rgba(23, 32, 51, 0.94);
  color: #d7dde8;
  padding: 2rem 0;
  font-family: var(--font-ui);
}

.footer-brand {
  margin: 0 0 0.35rem;
  color: #fff;
  font-weight: 700;
}

.footer-tagline,
.footer-copy {
  color: #b7c0ce;
  font-size: 0.95rem;
}

.footer-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem 1.25rem;
  margin: 1rem 0;
}

.footer-nav a {
  color: #9fd0cb;
  text-decoration: none;
  font-weight: 600;
}

.footer-nav a:hover,
.footer-nav a:focus-visible {
  text-decoration: underline;
  color: #fff;
}

.nowrap {
  white-space: nowrap;
}

@media (max-width: 980px) {
  :root {
    --shell: min(72rem, calc(100% - 1.5rem));
  }

  .layout-shell {
    gap: 0 1.5rem;
    grid-template-columns: 13.5rem minmax(0, 1fr);
  }
}

@media (max-width: 860px) {
  .sidebar-toggle {
    display: inline-block;
  }

  .layout-shell {
    grid-template-columns: 1fr;
    gap: 1rem;
    padding-top: 1.15rem;
  }

  .course-sidebar {
    position: static;
    max-height: none;
    display: none;
  }

  body.sidebar-open .course-sidebar {
    display: block;
  }

  main {
    max-width: none;
  }
}

@media (max-width: 640px) {
  .header-inner {
    min-height: 3.25rem;
  }

  h1 {
    font-size: clamp(1.65rem, 7vw, 2rem);
  }
}