/* ==========================================================================
   Learn Go - by Cyrus Mobini
   Main stylesheet. Uses CSS custom properties for light/dark theming.
   ========================================================================== */

/* ---------- Design tokens: shared ----------------------------------------- */
:root {
  --accent: #00ADD8;          /* Go brand cyan */
  --accent-700: #007d9c;      /* Go darker cyan */
  --accent-300: #5dc9e2;      /* Go lighter cyan */
  --accent-50:  #e6f7fb;

  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, .06);
  --shadow-md: 0 4px 16px rgba(0, 0, 0, .08);
  --shadow-lg: 0 20px 40px rgba(0, 0, 0, .12);

  --mono: ui-monospace, "SF Mono", "Cascadia Code", "JetBrains Mono",
          "Fira Code", Menlo, Consolas, monospace;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue",
          Arial, "Apple Color Emoji", "Segoe UI Emoji", sans-serif;

  --sidebar-w: 280px;
  --content-max: 760px;
  --header-h: 60px;

  --transition: 160ms cubic-bezier(.2, .8, .2, 1);
}

/* ---------- Light theme (default) ----------------------------------------- */
:root,
:root[data-theme="light"] {
  --bg:           #ffffff;
  --bg-elev:     #f7f9fb;
  --bg-muted:    #eef2f5;
  --fg:           #1a2330;
  --fg-muted:    #5a6a7e;
  --fg-subtle:   #8a98a8;
  --border:       #dde4ec;
  --border-strong:#c3ccd8;

  --code-bg:      #f5f8fa;
  --code-border:  #e1e7ee;
  --code-fg:      #2b3748;
  --inline-code-bg: #eef4f8;

  --callout-info-bg: #e6f7fb;
  --callout-info-br: #7bd3e8;
  --callout-warn-bg: #fff5e0;
  --callout-warn-br: #f0c36d;
  --callout-pitfall-bg: #ffe9e9;
  --callout-pitfall-br: #f0a0a0;
  --callout-tip-bg: #e8f6e9;
  --callout-tip-br: #8ac48f;

  --quiz-correct: #1f9a4f;
  --quiz-wrong:   #d1435b;
}

/* ---------- Dark theme ---------------------------------------------------- */
:root[data-theme="dark"] {
  --bg:           #0e141b;
  --bg-elev:     #151d26;
  --bg-muted:    #1c2733;
  --fg:           #e6edf3;
  --fg-muted:    #9ba9b9;
  --fg-subtle:   #6b7a8c;
  --border:       #27323f;
  --border-strong:#394856;

  --code-bg:      #0b1016;
  --code-border:  #1c2733;
  --code-fg:      #dfeaf3;
  --inline-code-bg: #1a232e;

  --callout-info-bg: #0a2a33;
  --callout-info-br: #1d6a80;
  --callout-warn-bg: #3a2e10;
  --callout-warn-br: #8a6a20;
  --callout-pitfall-bg: #3a1616;
  --callout-pitfall-br: #8a3a3a;
  --callout-tip-bg: #0f2e16;
  --callout-tip-br: #2e7a3b;

  --quiz-correct: #3fd47a;
  --quiz-wrong:   #ff6b81;

  --shadow-sm: 0 1px 2px rgba(0, 0, 0, .4);
  --shadow-md: 0 4px 16px rgba(0, 0, 0, .5);
  --shadow-lg: 0 20px 40px rgba(0, 0, 0, .6);
}

/* Respect system preference when no explicit choice is made */
@media (prefers-color-scheme: dark) {
  :root:not([data-theme]) {
    color-scheme: dark;
    --bg:           #0e141b;
    --bg-elev:     #151d26;
    --bg-muted:    #1c2733;
    --fg:           #e6edf3;
    --fg-muted:    #9ba9b9;
    --fg-subtle:   #6b7a8c;
    --border:       #27323f;
    --border-strong:#394856;
    --code-bg:      #0b1016;
    --code-border:  #1c2733;
    --code-fg:      #dfeaf3;
    --inline-code-bg: #1a232e;
    --callout-info-bg: #0a2a33;
    --callout-info-br: #1d6a80;
    --callout-warn-bg: #3a2e10;
    --callout-warn-br: #8a6a20;
    --callout-pitfall-bg: #3a1616;
    --callout-pitfall-br: #8a3a3a;
    --callout-tip-bg: #0f2e16;
    --callout-tip-br: #2e7a3b;
    --quiz-correct: #3fd47a;
    --quiz-wrong:   #ff6b81;
  }
}

/* ---------- Reset & base -------------------------------------------------- */
*, *::before, *::after { box-sizing: border-box; }

html { scroll-behavior: smooth; scroll-padding-top: calc(var(--header-h) + 12px); }

body {
  margin: 0;
  font-family: var(--sans);
  background: var(--bg);
  color: var(--fg);
  line-height: 1.65;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  transition: background var(--transition), color var(--transition);
}

img, svg { max-width: 100%; height: auto; display: block; }

a { color: var(--accent-700); text-decoration: none; }
a:hover { text-decoration: underline; }
:root[data-theme="dark"] a { color: var(--accent-300); }

h1, h2, h3, h4, h5, h6 {
  color: var(--fg);
  line-height: 1.25;
  margin-top: 1.8em;
  margin-bottom: .6em;
  font-weight: 650;
  letter-spacing: -0.01em;
}
h1 { font-size: 2.2rem; margin-top: 0; }
h2 { font-size: 1.6rem; border-bottom: 1px solid var(--border); padding-bottom: .3em; }
h3 { font-size: 1.25rem; }
h4 { font-size: 1.05rem; color: var(--fg-muted); }

p, ul, ol { margin: 0 0 1em; }
ul, ol { padding-left: 1.4em; }
li { margin-bottom: .3em; }

hr { border: 0; border-top: 1px solid var(--border); margin: 2em 0; }

blockquote {
  margin: 1em 0;
  padding: .6em 1em;
  border-left: 3px solid var(--accent);
  background: var(--bg-elev);
  color: var(--fg-muted);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
}

/* Inline code */
:not(pre) > code {
  font-family: var(--mono);
  font-size: .92em;
  padding: .12em .38em;
  background: var(--inline-code-bg);
  border-radius: 4px;
  color: var(--fg);
}

/* Tables */
table {
  width: 100%;
  border-collapse: collapse;
  margin: 1em 0;
  font-size: .95rem;
}
th, td {
  text-align: left;
  padding: .55em .8em;
  border-bottom: 1px solid var(--border);
}
th { background: var(--bg-elev); font-weight: 650; }
tbody tr:hover { background: var(--bg-elev); }

/* ---------- Site header --------------------------------------------------- */
.site-header {
  position: sticky;
  top: 0;
  z-index: 40;
  height: var(--header-h);
  background: color-mix(in srgb, var(--bg) 92%, transparent);
  backdrop-filter: saturate(180%) blur(8px);
  -webkit-backdrop-filter: saturate(180%) blur(8px);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  padding: 0 20px;
  gap: 16px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  color: var(--fg);
  font-size: 1.05rem;
  letter-spacing: -0.01em;
}
.brand:hover { text-decoration: none; }
.brand-mark {
  width: 30px; height: 30px;
  border-radius: 7px;
  background: linear-gradient(135deg, var(--accent), var(--accent-700));
  display: grid; place-items: center;
  color: white; font-weight: 800; font-family: var(--mono);
  box-shadow: var(--shadow-sm);
}
.brand-name { display: none; }
@media (min-width: 640px) { .brand-name { display: inline; } }
.brand-accent { color: var(--accent); }

.header-spacer { flex: 1; }

/* Header buttons */
.icon-btn {
  display: inline-grid;
  place-items: center;
  width: 38px;
  height: 38px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  background: var(--bg-elev);
  color: var(--fg);
  cursor: pointer;
  transition: background var(--transition), border-color var(--transition), color var(--transition);
}
.icon-btn:hover {
  background: var(--bg-muted);
  border-color: var(--border-strong);
}
.icon-btn svg { width: 18px; height: 18px; }

/* Mobile menu toggle hidden by default on desktop */
.menu-toggle { display: none; }
@media (max-width: 900px) { .menu-toggle { display: inline-grid; } }

/* ---------- Search -------------------------------------------------------- */
.search {
  position: relative;
  flex: 0 1 420px;
  max-width: 420px;
}
.search input {
  width: 100%;
  height: 38px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border);
  background: var(--bg-elev);
  color: var(--fg);
  padding: 0 12px 0 36px;
  font: inherit;
  transition: border-color var(--transition), box-shadow var(--transition);
}
.search input:focus {
  outline: none;
  border-color: var(--accent);
  box-shadow: 0 0 0 3px color-mix(in srgb, var(--accent) 25%, transparent);
}
.search::before {
  content: "";
  position: absolute;
  left: 12px; top: 50%;
  transform: translateY(-50%);
  width: 16px; height: 16px;
  background: no-repeat center/contain
    url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%238a98a8' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'><circle cx='11' cy='11' r='7'/><path d='m21 21-4.3-4.3'/></svg>");
  pointer-events: none;
}
.search-hint {
  position: absolute;
  right: 10px; top: 50%;
  transform: translateY(-50%);
  font-family: var(--mono);
  font-size: .75rem;
  color: var(--fg-subtle);
  padding: 2px 6px;
  border: 1px solid var(--border);
  border-radius: 4px;
  background: var(--bg);
  pointer-events: none;
}
.search-results {
  position: absolute;
  top: calc(100% + 8px);
  left: 0; right: 0;
  max-height: 60vh;
  overflow: auto;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  box-shadow: var(--shadow-lg);
  padding: 6px;
  z-index: 50;
  display: none;
}
.search-results.open { display: block; }
.search-result {
  display: block;
  padding: 10px 12px;
  border-radius: var(--radius-sm);
  color: var(--fg);
}
.search-result:hover,
.search-result.active {
  background: var(--bg-muted);
  text-decoration: none;
}
.search-result .sr-title { font-weight: 600; }
.search-result .sr-sub   { color: var(--fg-muted); font-size: .85em; margin-top: 2px; }
.search-result mark {
  background: color-mix(in srgb, var(--accent) 30%, transparent);
  color: inherit;
  padding: 0 2px;
  border-radius: 3px;
}
.search-empty {
  padding: 16px;
  color: var(--fg-muted);
  text-align: center;
  font-size: .9rem;
}

@media (max-width: 640px) {
  .search-hint { display: none; }
  .search { flex: 1; }
}

/* ---------- Layout: index page ------------------------------------------- */
.hero {
  padding: 80px 24px 60px;
  text-align: center;
  background:
    radial-gradient(circle at 20% 20%, color-mix(in srgb, var(--accent) 15%, transparent), transparent 40%),
    radial-gradient(circle at 80% 80%, color-mix(in srgb, var(--accent-300) 15%, transparent), transparent 40%);
  border-bottom: 1px solid var(--border);
}
.hero-inner {
  max-width: 760px;
  margin: 0 auto;
}
.hero h1 {
  font-size: clamp(2rem, 5vw, 3.2rem);
  font-weight: 750;
  letter-spacing: -0.02em;
  margin-bottom: .2em;
}
.hero .sub {
  font-size: 1.15rem;
  color: var(--fg-muted);
  max-width: 600px;
  margin: 0 auto 1.8em;
}
.hero-cta {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 12px 22px;
  border-radius: var(--radius-md);
  background: var(--accent);
  color: white!important;
  font-weight: 600;
  font-size: 1rem;
  transition: background var(--transition), transform var(--transition);
  box-shadow: var(--shadow-md);
}
.hero-cta:hover {
  background: var(--accent-700);
  text-decoration: none;
  transform: translateY(-1px);
}

.hero-stats {
  display: flex;
  justify-content: center;
  gap: 36px;
  flex-wrap: wrap;
  margin-top: 2em;
  color: var(--fg-muted);
  font-size: .95rem;
}
.hero-stats strong {
  display: block;
  color: var(--fg);
  font-size: 1.4rem;
  font-weight: 700;
}

/* Progress bar (global) */
.progress-wrap {
  max-width: 760px;
  margin: 40px auto 0;
  padding: 0 24px;
}
.progress-label {
  display: flex;
  justify-content: space-between;
  font-size: .9rem;
  color: var(--fg-muted);
  margin-bottom: 6px;
}
.progress-track {
  height: 8px;
  background: var(--bg-muted);
  border-radius: 999px;
  overflow: hidden;
}
.progress-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--accent-300), var(--accent));
  width: 0%;
  transition: width 400ms ease;
  border-radius: 999px;
}

/* TOC grid */
.toc {
  max-width: 1100px;
  margin: 60px auto;
  padding: 0 24px;
}
.toc h2 {
  text-align: center;
  border: 0;
  font-size: 1.9rem;
  margin-bottom: .2em;
}
.toc-sub {
  text-align: center;
  color: var(--fg-muted);
  margin-bottom: 2em;
}
.toc-part { margin-bottom: 40px; }
.toc-part:last-child { margin-bottom: 0; }
.toc-part-header {
  display: flex;
  align-items: baseline;
  gap: 14px;
  padding-bottom: 10px;
  margin-bottom: 16px;
  border-bottom: 1px solid var(--border);
}
.toc-part-num {
  font-family: var(--mono);
  font-size: .78rem;
  font-weight: 700;
  color: var(--accent);
  letter-spacing: .08em;
}
.toc-part-title {
  margin: 0;
  font-size: 1.3rem;
  font-weight: 650;
  color: var(--fg);
  letter-spacing: -0.01em;
}

.toc-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 14px;
}
.toc-card {
  display: flex;
  flex-direction: column;
  padding: 18px 20px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  color: var(--fg);
  transition: transform var(--transition), border-color var(--transition),
              box-shadow var(--transition), background var(--transition);
  position: relative;
  overflow: hidden;
}
.toc-card:hover {
  transform: translateY(-2px);
  border-color: var(--accent);
  box-shadow: var(--shadow-md);
  text-decoration: none;
}
.toc-card::before {
  content: "";
  position: absolute;
  inset: 0 auto 0 0;
  width: 3px;
  background: linear-gradient(180deg, var(--accent), var(--accent-700));
  opacity: 0;
  transition: opacity var(--transition);
}
.toc-card:hover::before { opacity: 1; }
.toc-card.done { border-color: color-mix(in srgb, var(--quiz-correct) 40%, var(--border)); }
.toc-card.done::after {
  content: "✓";
  position: absolute;
  top: 12px; right: 14px;
  width: 22px; height: 22px;
  border-radius: 50%;
  background: var(--quiz-correct);
  color: white;
  display: grid; place-items: center;
  font-size: .8rem;
  font-weight: 800;
}
.toc-num {
  font-family: var(--mono);
  font-size: .8rem;
  color: var(--accent);
  font-weight: 700;
  letter-spacing: .02em;
  margin-bottom: 6px;
}
.toc-title {
  font-size: 1.05rem;
  font-weight: 650;
  line-height: 1.3;
  margin-bottom: 6px;
}
.toc-desc {
  font-size: .88rem;
  color: var(--fg-muted);
  line-height: 1.45;
}

/* Sections on index */
.section-heading {
  max-width: 1100px;
  margin: 60px auto 24px;
  padding: 0 24px;
  font-size: 1.8rem;
  text-align: center;
  border: 0;
}

.features-grid {
  max-width: 1100px;
  margin: 0 auto 40px;
  padding: 0 24px;
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 14px;
}
.feature {
  padding: 20px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
}
.feature-icon {
  width: 36px; height: 36px;
  border-radius: 8px;
  background: color-mix(in srgb, var(--accent) 20%, transparent);
  color: var(--accent);
  display: grid; place-items: center;
  margin-bottom: 12px;
}
.feature-icon svg { width: 20px; height: 20px; }
.feature h3 { font-size: 1.05rem; margin: 0 0 6px; }
.feature p { font-size: .9rem; color: var(--fg-muted); margin: 0; }

/* ---------- Layout: chapter page ----------------------------------------- */
.chapter-layout {
  display: grid;
  grid-template-columns: var(--sidebar-w) 1fr;
  align-items: start;
  max-width: 1200px;
  margin: 0 auto;
  min-height: calc(100vh - var(--header-h) - 80px);
}
@media (max-width: 900px) {
  .chapter-layout { grid-template-columns: 1fr; }
}

.chapter-sidebar {
  position: sticky;
  top: var(--header-h);
  max-height: calc(100vh - var(--header-h));
  overflow-y: auto;
  padding: 28px 20px;
  border-right: 1px solid var(--border);
  font-size: .93rem;
}
@media (max-width: 900px) {
  .chapter-sidebar {
    position: fixed;
    inset: var(--header-h) auto 0 0;
    width: 280px;
    max-width: 85vw;
    background: var(--bg);
    border-right: 1px solid var(--border);
    transform: translateX(-100%);
    transition: transform var(--transition);
    z-index: 30;
    box-shadow: var(--shadow-lg);
  }
  .chapter-sidebar.open { transform: translateX(0); }
}
.chapter-sidebar .side-title {
  font-size: .75rem;
  text-transform: uppercase;
  letter-spacing: .08em;
  color: var(--fg-subtle);
  margin-bottom: 8px;
  font-weight: 700;
}
.chapter-sidebar ul { list-style: none; padding: 0; margin: 0 0 24px; }
.chapter-sidebar li { margin: 0; }
.chapter-sidebar a {
  display: block;
  padding: 6px 10px;
  border-radius: 6px;
  color: var(--fg-muted);
  border-left: 2px solid transparent;
  transition: color var(--transition), background var(--transition), border-color var(--transition);
}
.chapter-sidebar a:hover { color: var(--fg); background: var(--bg-elev); text-decoration: none; }
.chapter-sidebar a.active {
  color: var(--accent);
  background: color-mix(in srgb, var(--accent) 8%, transparent);
  border-left-color: var(--accent);
  font-weight: 600;
}
.chapter-sidebar ul ul a { padding-left: 24px; font-size: .89rem; }

.chapter-main {
  padding: 40px 36px 60px;
  max-width: calc(var(--content-max) + 72px);
  width: 100%;
  justify-self: center;
  min-width: 0;
}
@media (max-width: 900px) {
  .chapter-main { padding: 24px 20px 40px; }
}

.chapter-eyebrow {
  display: flex;
  align-items: center;
  gap: 12px;
  font-family: var(--mono);
  font-size: .85rem;
  color: var(--accent);
  font-weight: 700;
  margin-bottom: 8px;
}
.chapter-eyebrow .dot {
  width: 6px; height: 6px; border-radius: 50%;
  background: var(--accent);
}

/* Learning objectives box */
.learning-objectives {
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 4px solid var(--accent);
  padding: 18px 22px;
  border-radius: var(--radius-md);
  margin: 24px 0 32px;
}
.learning-objectives h3 {
  margin: 0 0 .4em;
  font-size: 1rem;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: .05em;
}
.learning-objectives ul { margin: 0; padding-left: 1.2em; }
.learning-objectives li { color: var(--fg); font-size: .95rem; }

/* Callouts */
.callout {
  display: grid;
  grid-template-columns: 28px 1fr;
  gap: 12px;
  padding: 14px 18px;
  border-radius: var(--radius-md);
  border: 1px solid;
  margin: 1.2em 0;
  align-items: start;
}
.callout .ico {
  width: 24px; height: 24px;
  border-radius: 50%;
  display: grid; place-items: center;
  font-weight: 800;
  font-size: .9rem;
  color: white;
}
.callout p:last-child, .callout ul:last-child, .callout ol:last-child { margin-bottom: 0; }
.callout-title {
  font-weight: 700;
  margin-bottom: 4px;
  display: block;
}
.callout.info    { background: var(--callout-info-bg);    border-color: var(--callout-info-br); }
.callout.info    .ico { background: #0b8ba9; }
.callout.warn    { background: var(--callout-warn-bg);    border-color: var(--callout-warn-br); }
.callout.warn    .ico { background: #c98a1a; }
.callout.pitfall { background: var(--callout-pitfall-bg); border-color: var(--callout-pitfall-br); }
.callout.pitfall .ico { background: #c13a3a; }
.callout.tip     { background: var(--callout-tip-bg);     border-color: var(--callout-tip-br); }
.callout.tip     .ico { background: #2e8a3e; }

/* Code blocks */
.code-block {
  position: relative;
  margin: 1.2em 0;
  border: 1px solid var(--code-border);
  border-radius: var(--radius-md);
  background: var(--code-bg);
  overflow: hidden;
}
.code-block-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 8px 12px;
  background: color-mix(in srgb, var(--code-bg) 80%, var(--bg-muted));
  border-bottom: 1px solid var(--code-border);
  font-family: var(--mono);
  font-size: .78rem;
  color: var(--fg-muted);
}
.code-block-lang {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  text-transform: lowercase;
}
.code-block-lang::before {
  content: "";
  width: 8px; height: 8px;
  border-radius: 50%;
  background: var(--accent);
}
.code-block-actions { display: flex; gap: 6px; }
.code-btn {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 4px 10px;
  border-radius: 5px;
  border: 1px solid var(--border);
  background: var(--bg);
  color: var(--fg);
  font-size: .78rem;
  font-family: var(--sans);
  cursor: pointer;
  transition: background var(--transition), border-color var(--transition), color var(--transition);
}
.code-btn:hover {
  background: var(--bg-muted);
  border-color: var(--border-strong);
}
.code-btn svg { width: 12px; height: 12px; }
.code-btn.copied { color: var(--quiz-correct); border-color: var(--quiz-correct); }
.code-btn.run { color: var(--accent); border-color: color-mix(in srgb, var(--accent) 50%, var(--border)); }
.code-btn.run:hover { background: color-mix(in srgb, var(--accent) 12%, var(--bg)); }

.code-block pre {
  margin: 0;
  padding: 16px 18px;
  overflow-x: auto;
  font-family: var(--mono);
  font-size: .9rem;
  line-height: 1.55;
  color: var(--code-fg);
  tab-size: 4;
}
.code-block pre code {
  font-family: inherit;
  background: transparent;
  padding: 0;
  font-size: inherit;
  color: inherit;
}

/* Diagrams */
.diagram {
  margin: 1.5em 0;
  padding: 20px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  text-align: center;
}
.diagram svg { margin: 0 auto; }
.diagram-caption {
  margin-top: 10px;
  font-size: .85rem;
  color: var(--fg-muted);
  font-style: italic;
}

/* Quiz */
.quiz {
  margin: 2.5em 0;
  padding: 26px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
}
.quiz-header { display: flex; align-items: center; gap: 10px; margin-bottom: 18px; }
.quiz-header h3 { margin: 0; font-size: 1.2rem; }
.quiz-header .badge {
  font-family: var(--mono);
  font-size: .75rem;
  padding: 3px 9px;
  background: color-mix(in srgb, var(--accent) 18%, transparent);
  color: var(--accent);
  border-radius: 999px;
  font-weight: 700;
}
.quiz-question { margin-bottom: 20px; padding-bottom: 20px; border-bottom: 1px dashed var(--border); }
.quiz-question:last-of-type { border-bottom: 0; padding-bottom: 0; margin-bottom: 10px; }
.quiz-question .q-text {
  font-weight: 600;
  margin-bottom: 10px;
  color: var(--fg);
}
.quiz-question .q-num {
  display: inline-block;
  font-family: var(--mono);
  font-size: .8rem;
  color: var(--accent);
  margin-right: 8px;
}
.quiz-option {
  display: block;
  padding: 10px 14px;
  margin-bottom: 6px;
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
  cursor: pointer;
  background: var(--bg);
  transition: background var(--transition), border-color var(--transition);
  font-size: .95rem;
  user-select: none;
}
.quiz-option:hover { background: var(--bg-muted); border-color: var(--border-strong); }
.quiz-option input { margin-right: 10px; accent-color: var(--accent); }
.quiz-option.correct {
  background: color-mix(in srgb, var(--quiz-correct) 15%, var(--bg));
  border-color: var(--quiz-correct);
}
.quiz-option.wrong {
  background: color-mix(in srgb, var(--quiz-wrong) 15%, var(--bg));
  border-color: var(--quiz-wrong);
}
.quiz-option.disabled { cursor: default; }
.quiz-explanation {
  display: none;
  margin-top: 10px;
  padding: 10px 14px;
  background: var(--bg);
  border-left: 3px solid var(--accent);
  border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
  font-size: .9rem;
  color: var(--fg-muted);
}
.quiz-explanation.show { display: block; }
.quiz-explanation strong { color: var(--fg); }
.quiz-summary {
  margin-top: 16px;
  padding: 14px;
  text-align: center;
  background: var(--bg);
  border-radius: var(--radius-sm);
  font-weight: 600;
}

/* Exercise */
.exercise {
  margin: 2em 0;
  padding: 24px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-left: 4px solid var(--accent);
  border-radius: var(--radius-md);
}
.exercise-header { display: flex; align-items: center; gap: 10px; margin-bottom: 10px; }
.exercise-header h3 { margin: 0; font-size: 1.15rem; }
.exercise-header .badge {
  font-family: var(--mono);
  font-size: .72rem;
  padding: 3px 8px;
  background: var(--accent);
  color: white;
  border-radius: 4px;
  font-weight: 700;
  letter-spacing: .03em;
}
.exercise-task p:first-child { margin-top: 0; }
.exercise-solution {
  margin-top: 16px;
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: var(--radius-sm);
}
.exercise-solution summary {
  padding: 10px 14px;
  cursor: pointer;
  font-weight: 600;
  color: var(--accent);
  user-select: none;
}
.exercise-solution[open] summary { border-bottom: 1px solid var(--border); }
.exercise-solution > *:not(summary) { padding: 12px 16px; }
.exercise-solution .code-block { margin: 0; border-radius: 0; border-left: 0; border-right: 0; border-top: 0; }

/* Chapter nav (prev/next) */
.chapter-nav {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 14px;
  margin: 3em 0 1em;
}
@media (max-width: 560px) { .chapter-nav { grid-template-columns: 1fr; } }
.chapter-nav a {
  display: block;
  padding: 16px 20px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
  transition: border-color var(--transition), transform var(--transition), background var(--transition);
  color: var(--fg);
}
.chapter-nav a:hover {
  border-color: var(--accent);
  transform: translateY(-1px);
  text-decoration: none;
  background: color-mix(in srgb, var(--accent) 5%, var(--bg-elev));
}
.chapter-nav .nav-label { font-size: .75rem; color: var(--fg-subtle); text-transform: uppercase; letter-spacing: .06em; }
.chapter-nav .nav-title { font-weight: 650; margin-top: 4px; color: var(--fg); }
.chapter-nav .prev { text-align: left; }
.chapter-nav .next { text-align: right; }
.chapter-nav .nav-label::before,
.chapter-nav .nav-label::after { font-family: var(--mono); }
.chapter-nav .prev .nav-label::before { content: "← "; }
.chapter-nav .next .nav-label::after  { content: " →"; }

/* Mark complete */
.complete-box {
  margin: 2em 0 1em;
  padding: 22px;
  background: linear-gradient(135deg,
    color-mix(in srgb, var(--accent) 10%, var(--bg-elev)),
    var(--bg-elev));
  border: 1px solid var(--accent);
  border-radius: var(--radius-md);
  text-align: center;
}
.complete-box.done {
  background: color-mix(in srgb, var(--quiz-correct) 10%, var(--bg-elev));
  border-color: var(--quiz-correct);
}
.complete-box p { margin: 0 0 12px; color: var(--fg-muted); }
.complete-btn {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 10px 22px;
  background: var(--accent);
  color: white;
  border: 0;
  border-radius: var(--radius-sm);
  font: inherit;
  font-weight: 650;
  cursor: pointer;
  transition: background var(--transition), transform var(--transition);
}
.complete-btn:hover { background: var(--accent-700); transform: translateY(-1px); }
.complete-btn.done { background: var(--quiz-correct); }

/* Further reading */
.further-reading {
  margin: 2em 0 1em;
  padding: 20px 24px;
  background: var(--bg-elev);
  border: 1px solid var(--border);
  border-radius: var(--radius-md);
}
.further-reading h3 {
  margin: 0 0 .5em;
  font-size: 1rem;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: .05em;
}
.further-reading ul { margin: 0; padding-left: 1.3em; }
.further-reading li { margin-bottom: 4px; font-size: .95rem; }

/* ---------- Footer -------------------------------------------------------- */
.site-footer {
  margin-top: 80px;
  padding: 36px 24px;
  text-align: center;
  color: var(--fg-muted);
  border-top: 1px solid var(--border);
  background: var(--bg-elev);
  font-size: .92rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 14px;
}
.site-footer a { color: var(--accent); font-weight: 600; }
.site-footer .heart { color: #e74c3c; display: inline-block; }
.footer-line { line-height: 1.6; }

.bmac {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 9px 18px;
  background: #FFDD00;
  color: #1a2330 !important;
  border-radius: 8px;
  font-weight: 700;
  font-size: .92rem;
  text-decoration: none;
  border: 1px solid #e6c700;
  box-shadow: 0 1px 2px rgba(0, 0, 0, .08);
  transition: transform var(--transition), box-shadow var(--transition), filter var(--transition);
}
.bmac:hover {
  text-decoration: none;
  transform: translateY(-1px);
  box-shadow: 0 4px 12px rgba(0, 0, 0, .15);
  filter: brightness(1.04);
}
.bmac-icon {
  font-size: 1.1em;
  line-height: 1;
}

/* ---------- Utilities ----------------------------------------------------- */
.visually-hidden {
  position: absolute;
  width: 1px; height: 1px;
  padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0);
  white-space: nowrap;
  border: 0;
}

.backdrop {
  display: none;
  position: fixed;
  inset: var(--header-h) 0 0 0;
  background: rgba(0, 0, 0, .4);
  z-index: 25;
}
.backdrop.open { display: block; }
@media (min-width: 901px) { .backdrop.open { display: none; } }

/* Smooth anchor target highlight */
:target {
  animation: target-flash 1.4s ease-out;
}
@keyframes target-flash {
  0%   { background: color-mix(in srgb, var(--accent) 25%, transparent); }
  100% { background: transparent; }
}

/* Scrollbars (webkit) */
::-webkit-scrollbar { width: 10px; height: 10px; }
::-webkit-scrollbar-track { background: var(--bg-muted); }
::-webkit-scrollbar-thumb { background: var(--border-strong); border-radius: 10px; }
::-webkit-scrollbar-thumb:hover { background: var(--fg-subtle); }