@import url('https://fonts.googleapis.com/css2?family=IBM+Plex+Sans+JP:wght@400;500;600;700&family=JetBrains+Mono:wght@400;500;600&display=swap');

:root {
  --bg: #000000;
  --bg-elevated: #0a0a0a;
  --bg-sidebar: #050505;
  --bg-code: #161616;
  --bg-code-header: #1c1c1c;
  --bg-inline: #2a2a2a;
  --border: #2a2a2a;
  --border-soft: #1f1f1f;
  --text: #f5f5f5;
  --text-muted: #a3a3a3;
  --text-dim: #737373;
  --accent: #a3a3a3;
  --callout-bar: #525252;
  --link: #e5e5e5;
  --link-hover: #ffffff;
  --kw: #c9a227;
  --fn: #7eb6ff;
  --str: #9ece6a;
  --cm: #6b7280;
  --num: #f97583;
  --op: #e5e5e5;
  --radius: 10px;
  --sidebar-w: 280px;
  --content-max: 720px;
  --font: 'IBM Plex Sans JP', sans-serif;
  --mono: 'JetBrains Mono', ui-monospace, monospace;
}

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

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: var(--font);
  background: var(--bg);
  color: var(--text);
  line-height: 1.85;
  font-size: 16px;
  -webkit-font-smoothing: antialiased;
}

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

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

/* Layout */
.app {
  display: flex;
  min-height: 100vh;
}

.sidebar {
  position: fixed;
  top: 0;
  left: 0;
  width: var(--sidebar-w);
  height: 100vh;
  overflow-y: auto;
  background: var(--bg-sidebar);
  border-right: 1px solid var(--border-soft);
  padding: 1.25rem 0 3rem;
  z-index: 40;
}

.sidebar::-webkit-scrollbar {
  width: 6px;
}

.sidebar::-webkit-scrollbar-thumb {
  background: #333;
  border-radius: 3px;
}

.brand {
  display: block;
  padding: 0.5rem 1.25rem 1.25rem;
  border-bottom: 1px solid var(--border-soft);
  margin-bottom: 0.75rem;
}

.brand-title {
  font-size: 0.95rem;
  font-weight: 700;
  letter-spacing: 0.02em;
  line-height: 1.4;
  color: #fff;
}

.brand-sub {
  margin-top: 0.25rem;
  font-size: 0.75rem;
  color: var(--text-dim);
}

.nav-chapter {
  margin-bottom: 0.15rem;
}

.nav-chapter-btn {
  width: 100%;
  display: flex;
  align-items: flex-start;
  gap: 0.5rem;
  padding: 0.55rem 1.25rem;
  background: transparent;
  border: 0;
  color: var(--text-muted);
  font-family: inherit;
  font-size: 0.82rem;
  font-weight: 500;
  text-align: left;
  cursor: pointer;
  line-height: 1.45;
}

.nav-chapter-btn:hover,
.nav-chapter.active > .nav-chapter-btn {
  color: #fff;
  background: #111;
}

.nav-num {
  flex-shrink: 0;
  color: var(--text-dim);
  font-variant-numeric: tabular-nums;
}

.nav-sections {
  display: none;
  padding: 0.15rem 0 0.5rem;
}

.nav-chapter.open .nav-sections,
.nav-chapter.active .nav-sections {
  display: block;
}

.nav-sections a {
  display: block;
  padding: 0.35rem 1.25rem 0.35rem 2.4rem;
  font-size: 0.78rem;
  color: var(--text-dim);
  line-height: 1.4;
}

.nav-sections a:hover,
.nav-sections a.active {
  color: #fff;
}

.main {
  margin-left: var(--sidebar-w);
  flex: 1;
  min-width: 0;
}

.topbar {
  position: sticky;
  top: 0;
  z-index: 30;
  display: none;
  align-items: center;
  gap: 0.75rem;
  padding: 0.75rem 1rem;
  background: rgba(0, 0, 0, 0.9);
  border-bottom: 1px solid var(--border-soft);
  backdrop-filter: blur(8px);
}

.menu-btn {
  background: #161616;
  border: 1px solid var(--border);
  color: #fff;
  border-radius: 8px;
  padding: 0.4rem 0.7rem;
  font-family: inherit;
  cursor: pointer;
}

.content {
  max-width: var(--content-max);
  margin: 0 auto;
  padding: 2.5rem 1.5rem 6rem;
}

/* Home */
.hero {
  padding: 1rem 0 2.5rem;
}

.hero-label {
  font-size: 0.8rem;
  color: var(--text-dim);
  letter-spacing: 0.08em;
  text-transform: uppercase;
  margin-bottom: 0.75rem;
}

.hero h1 {
  margin: 0 0 1rem;
  font-size: clamp(1.75rem, 4vw, 2.4rem);
  font-weight: 700;
  letter-spacing: -0.02em;
  line-height: 1.35;
}

.hero p {
  margin: 0;
  color: var(--text-muted);
  font-size: 1.05rem;
  max-width: 36em;
}

.toc-grid {
  display: grid;
  gap: 0.75rem;
  margin-top: 2.5rem;
}

.toc-card {
  display: block;
  padding: 1.15rem 1.25rem;
  border: 1px solid var(--border-soft);
  border-radius: var(--radius);
  background: linear-gradient(180deg, #0c0c0c 0%, #080808 100%);
  transition: border-color 0.15s ease, background 0.15s ease;
}

.toc-card:hover {
  border-color: #404040;
  background: #101010;
  color: inherit;
}

.toc-card-num {
  font-size: 0.75rem;
  color: var(--text-dim);
  margin-bottom: 0.35rem;
}

.toc-card h2 {
  margin: 0 0 0.4rem;
  font-size: 1.05rem;
  font-weight: 600;
}

.toc-card p {
  margin: 0;
  font-size: 0.88rem;
  color: var(--text-muted);
  line-height: 1.6;
}

.toc-note {
  display: inline-block;
  margin-top: 0.55rem;
  font-size: 0.78rem;
  color: #d4d4d4;
  border-left: 2px solid var(--callout-bar);
  padding-left: 0.65rem;
}

/* Lesson typography */
.chapter-head {
  margin-bottom: 2.5rem;
  padding-bottom: 1.5rem;
  border-bottom: 1px solid var(--border-soft);
}

.chapter-head .eyebrow {
  font-size: 0.8rem;
  color: var(--text-dim);
  margin-bottom: 0.5rem;
}

.chapter-head h1 {
  margin: 0 0 0.75rem;
  font-size: clamp(1.6rem, 3.5vw, 2.1rem);
  font-weight: 700;
  line-height: 1.35;
}

.chapter-head .lead {
  margin: 0;
  color: var(--text-muted);
}

.lesson {
  margin-bottom: 3.5rem;
  scroll-margin-top: 1.5rem;
}

.lesson h2 {
  margin: 0 0 1.25rem;
  font-size: 1.45rem;
  font-weight: 700;
  line-height: 1.45;
}

.lesson h3 {
  margin: 2rem 0 0.85rem;
  font-size: 1.1rem;
  font-weight: 600;
}

.lesson p {
  margin: 0 0 1.1rem;
  color: #e8e8e8;
}

.lesson ul,
.lesson ol {
  margin: 0 0 1.25rem;
  padding-left: 1.35rem;
  color: #e8e8e8;
}

.lesson li {
  margin-bottom: 0.4rem;
}

.lesson li::marker {
  color: var(--text-dim);
}

/* Inline code */
:not(pre) > code,
.inline-code {
  font-family: var(--mono);
  font-size: 0.86em;
  background: var(--bg-inline);
  color: #f0f0f0;
  padding: 0.12em 0.4em;
  border-radius: 5px;
  font-weight: 500;
}

h2 code,
h3 code {
  font-size: 0.88em;
}

/* Callout / definition bar */
.callout {
  margin: 1.25rem 0 1.5rem;
  padding: 0.15rem 0 0.15rem 1rem;
  border-left: 3px solid var(--callout-bar);
  color: #f0f0f0;
  font-size: 1.02rem;
  line-height: 1.75;
}

.callout strong {
  font-weight: 600;
}

/* Beginner helpers */
.tip,
.note-box,
.warn,
.analogy {
  margin: 1.25rem 0 1.5rem;
  padding: 1rem 1.1rem;
  border-radius: 8px;
  border: 1px solid #2a2a2a;
  background: #0c0c0c;
}

.tip-label,
.note-label,
.warn-label,
.analogy-label {
  display: block;
  font-size: 0.72rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  color: var(--text-dim);
  margin-bottom: 0.45rem;
}

.tip p,
.note-box p,
.warn p,
.analogy p,
.tip ul,
.note-box ul,
.warn ul,
.analogy ul {
  margin: 0;
}

.tip ul,
.note-box ul,
.warn ul,
.analogy ul {
  padding-left: 1.2rem;
  margin-top: 0.5rem;
}

.tip {
  border-color: #2a3540;
  background: #0a1016;
}

.warn {
  border-color: #3a2a2a;
  background: #120c0c;
}

.analogy {
  border-color: #2a2a32;
  background: #0c0c12;
}

.line-explain {
  margin: 0.75rem 0 1.35rem;
  padding: 0;
  list-style: none;
}

.line-explain li {
  display: grid;
  grid-template-columns: minmax(7rem, 11rem) 1fr;
  gap: 0.75rem;
  padding: 0.55rem 0;
  border-bottom: 1px solid #1a1a1a;
  font-size: 0.92rem;
}

.line-explain li:last-child {
  border-bottom: 0;
}

.line-explain code {
  align-self: start;
}

.line-explain span {
  color: var(--text-muted);
}

@media (max-width: 600px) {
  .line-explain li {
    grid-template-columns: 1fr;
    gap: 0.25rem;
  }
}

/* Code block */
.code-block {
  margin: 1.35rem 0 1.6rem;
  border-radius: var(--radius);
  background: var(--bg-code);
  border: 1px solid #222;
  overflow: hidden;
}

.code-block-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.55rem 0.85rem;
  background: var(--bg-code-header);
  border-bottom: 1px solid #222;
}

.code-lang {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  font-family: var(--mono);
  font-size: 0.75rem;
  color: var(--text-muted);
}

.code-lang svg {
  width: 14px;
  height: 14px;
  opacity: 0.7;
}

.copy-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 32px;
  padding: 0;
  background: transparent;
  border: 0;
  border-radius: 6px;
  color: var(--text-dim);
  cursor: pointer;
}

.copy-btn:hover {
  color: #fff;
  background: #2a2a2a;
}

.copy-btn.copied {
  color: #9ece6a;
}

.code-block pre {
  margin: 0;
  padding: 1rem 1.1rem 1.15rem;
  overflow-x: auto;
  font-family: var(--mono);
  font-size: 0.84rem;
  line-height: 1.7;
}

.code-block code {
  font-family: inherit;
  background: transparent;
  padding: 0;
  color: #e8e8e8;
}

/* Simple syntax tokens */
.tok-kw { color: var(--kw); }
.tok-fn { color: var(--fn); }
.tok-str { color: var(--str); }
.tok-cm { color: var(--cm); font-style: italic; }
.tok-num { color: var(--num); }
.tok-var { color: #e8e8e8; }
.tok-op { color: var(--op); }

/* Exercise box */
.exercise {
  margin: 1.5rem 0;
  padding: 1.25rem 1.35rem;
  border: 1px solid #2e2e2e;
  border-radius: var(--radius);
  background: #0c0c0c;
}

.exercise-label {
  font-size: 0.75rem;
  font-weight: 600;
  letter-spacing: 0.06em;
  color: var(--text-dim);
  margin-bottom: 0.65rem;
}

.exercise h3 {
  margin: 0 0 0.75rem;
  font-size: 1.05rem;
}

.exercise ol {
  margin-bottom: 0;
}

/* Pager */
.pager {
  display: flex;
  justify-content: space-between;
  gap: 1rem;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-soft);
}

.pager a {
  display: inline-flex;
  flex-direction: column;
  gap: 0.2rem;
  padding: 0.85rem 1rem;
  border: 1px solid var(--border-soft);
  border-radius: var(--radius);
  min-width: 0;
  flex: 1;
  background: #0a0a0a;
}

.pager a:hover {
  border-color: #404040;
  color: #fff;
}

.pager .dir {
  font-size: 0.72rem;
  color: var(--text-dim);
}

.pager .title {
  font-size: 0.9rem;
  font-weight: 500;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.pager a.next {
  text-align: right;
  align-items: flex-end;
}

/* Overlay for mobile */
.sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.6);
  z-index: 35;
}

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

@media (max-width: 900px) {
  .sidebar {
    transform: translateX(-100%);
    transition: transform 0.2s ease;
  }

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

  .main {
    margin-left: 0;
  }

  .topbar {
    display: flex;
  }

  .content {
    padding: 1.5rem 1.1rem 4rem;
  }
}