:root {
  --lesson-accent: #149ddd;
  --lesson-accent-dark: #087cad;
  --lesson-navy: #07111f;
  --lesson-ink: #162536;
  --lesson-muted: #5f7083;
  --lesson-surface: #ffffff;
  --lesson-soft: #f3f7fb;
  --lesson-border: #d6e0ea;
  --lesson-font-scale: 1;
  --lesson-shadow: 0 16px 45px rgba(19, 40, 63, .09);
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  background: radial-gradient(circle at top right, #e8f6fd 0, transparent 33rem), var(--lesson-soft);
  color: var(--lesson-ink);
  font-family: "Roboto", "Segoe UI", Arial, sans-serif;
  font-size: calc(1rem * var(--lesson-font-scale));
  line-height: 1.65;
}
a { color: var(--lesson-accent-dark); }
a:focus-visible, button:focus-visible, [tabindex]:focus-visible, summary:focus-visible {
  outline: 3px solid #f2b705;
  outline-offset: 3px;
}

.material-top {
  align-items: center;
  background: rgba(7, 17, 31, .97);
  color: #fff;
  display: flex;
  justify-content: space-between;
  min-height: 68px;
  padding: .75rem clamp(1rem, 4vw, 3rem);
  position: sticky;
  top: 0;
  z-index: 1000;
}
.material-top a { color: #fff; text-decoration: none; }
.material-brand { align-items: center; display: flex; gap: .8rem; font-weight: 700; }
.material-brand-mark {
  background: var(--lesson-accent);
  border-radius: .75rem;
  display: grid;
  height: 42px;
  place-items: center;
  width: 42px;
}
.material-top-links { display: flex; gap: .75rem; }
.material-top-links a { border: 1px solid rgba(255,255,255,.3); border-radius: .6rem; padding: .45rem .75rem; }
.material-shell { margin: auto; max-width: 1540px; padding: clamp(1.2rem, 3vw, 2.6rem); }

.library-hero, .lesson-hero {
  background: linear-gradient(125deg, #fff 0%, #f4fbff 65%, #e1f5fd 100%);
  border: 1px solid #d7eaf4;
  border-radius: 1.35rem;
  box-shadow: var(--lesson-shadow);
  margin-bottom: 1.6rem;
  overflow: hidden;
  padding: clamp(1.5rem, 4vw, 3.4rem);
  position: relative;
}
.library-hero::after, .lesson-hero::after {
  background: var(--lesson-accent);
  border-radius: 999px;
  content: "";
  height: 190px;
  opacity: .1;
  position: absolute;
  right: -60px;
  top: -70px;
  width: 190px;
}
.eyebrow { color: var(--lesson-accent-dark); font-size: .78rem; font-weight: 800; letter-spacing: .1em; text-transform: uppercase; }
.library-hero h1, .lesson-hero h1 { font-size: clamp(2rem, 5vw, 3.65rem); letter-spacing: -.045em; line-height: 1.08; margin: .45rem 0 1rem; max-width: 1050px; }
.library-hero .lead, .lesson-hero .lead { color: var(--lesson-muted); max-width: 900px; }
.library-grid { display: grid; gap: 1.4rem; grid-template-columns: repeat(auto-fit, minmax(310px, 1fr)); }
.library-card {
  background: var(--lesson-surface);
  border: 1px solid var(--lesson-border);
  border-radius: 1.2rem;
  box-shadow: var(--lesson-shadow);
  padding: 1.6rem;
}
.library-card h2 { font-size: 1.7rem; }
.library-card section { border-top: 1px solid var(--lesson-border); margin-top: 1.25rem; padding-top: 1.25rem; }

.lesson-toolbar {
  align-items: center;
  background: #fff;
  border: 1px solid var(--lesson-border);
  border-radius: 1rem;
  display: flex;
  flex-wrap: wrap;
  gap: .75rem;
  justify-content: space-between;
  margin-top: 1.4rem;
  padding: .85rem 1rem;
  position: relative;
  z-index: 1;
}
.lesson-progress { flex: 1 1 260px; }
.lesson-progress-track { background: #dfe9ef; border-radius: 99px; height: 8px; margin-top: .35rem; overflow: hidden; }
.lesson-progress-bar { background: linear-gradient(90deg, var(--lesson-accent-dark), var(--lesson-accent)); border-radius: inherit; height: 100%; transition: width .25s ease; width: 10%; }
.lesson-tools { display: flex; flex-wrap: wrap; gap: .45rem; }
.lesson-tools button, .hierarchy-actions button {
  background: #fff;
  border: 1px solid #b9c9d6;
  border-radius: .55rem;
  color: var(--lesson-ink);
  min-height: 40px;
  padding: .4rem .7rem;
}
.lesson-tools button:hover, .hierarchy-actions button:hover { background: #edf8fd; border-color: var(--lesson-accent); }

.lesson-layout { display: grid; gap: 1.5rem; grid-template-columns: 300px minmax(0, 1fr); }
.step-nav {
  align-self: start;
  background: rgba(255,255,255,.96);
  border: 1px solid var(--lesson-border);
  border-radius: 1rem;
  box-shadow: var(--lesson-shadow);
  max-height: calc(100vh - 100px);
  overflow: auto;
  padding: .9rem;
  position: sticky;
  top: 88px;
}
.step-nav-title { display: block; padding: .45rem .65rem .75rem; }
.step-nav a {
  align-items: flex-start;
  border-left: 3px solid transparent;
  border-radius: .6rem;
  color: #43566a;
  display: flex;
  gap: .65rem;
  line-height: 1.35;
  margin: .1rem 0;
  padding: .65rem;
  text-decoration: none;
}
.step-nav a:hover { background: #f1f8fc; }
.step-nav a.active { background: #e5f5fc; border-left-color: var(--lesson-accent); color: #075f87; font-weight: 700; }
.step-number { background: #e8eef4; border-radius: .4rem; color: #43566a; flex: 0 0 auto; font-size: .75rem; font-weight: 800; padding: .16rem .32rem; }
.step-nav a.active .step-number { background: var(--lesson-accent); color: #fff; }
.lesson-canvas { min-width: 0; }
.lesson-step {
  background: var(--lesson-surface);
  border: 1px solid var(--lesson-border);
  border-radius: 1.25rem;
  box-shadow: var(--lesson-shadow);
  margin-bottom: 1.5rem;
  padding: clamp(1.25rem, 3vw, 2.5rem);
  scroll-margin-top: 90px;
}
.js .lesson-step[hidden] { display: none; }
.lesson-step > h2 { font-size: clamp(1.7rem, 3.5vw, 2.55rem); letter-spacing: -.03em; line-height: 1.2; margin-bottom: 1.2rem; }
.lesson-step-count { color: var(--lesson-accent-dark); font-size: .8rem; font-weight: 800; letter-spacing: .07em; text-transform: uppercase; }
.lesson-objective, .lesson-takeaway, .lesson-check {
  background: #eff9fd;
  border-left: 4px solid var(--lesson-accent);
  border-radius: 0 .8rem .8rem 0;
  margin: 1.15rem 0;
  padding: 1rem 1.15rem;
}
.lesson-objective strong, .lesson-takeaway strong { display: block; margin-bottom: .2rem; }
.lesson-takeaway { background: #eef9f1; border-left-color: #2a9a4a; }
.lesson-check { background: #fff9e9; border-left-color: #e6a700; }
.lesson-check summary { cursor: pointer; }
.lesson-block { margin: 1.5rem 0; }
.lesson-prose h3, .lesson-list h3, .lesson-note h3 { font-size: 1.15rem; margin-bottom: .6rem; }
.lesson-list ul { display: grid; gap: .55rem; padding-left: 1.35rem; }
.lesson-note { border-radius: .8rem; padding: 1rem 1.15rem; }
.lesson-note-important { background: #edf8fd; border: 1px solid #b9e0f2; }
.lesson-note-warning { background: #fff7df; border: 1px solid #efd78c; }
.lesson-note-tip { background: #edf8ef; border: 1px solid #b8dfc0; }
.lesson-block-unavailable { background: #fff2f2; border: 1px solid #e8b8b8; border-radius: .7rem; color: #7b2727; padding: 1rem; }

.lesson-table-wrap { overflow-x: auto; }
.lesson-table { border-collapse: separate; border-spacing: 0; min-width: 650px; width: 100%; }
.lesson-table caption { color: var(--lesson-ink); font-size: 1.12rem; font-weight: 800; padding: 0 0 .7rem; text-align: left; }
.lesson-table th, .lesson-table td { border-bottom: 1px solid var(--lesson-border); padding: .78rem .85rem; text-align: left; vertical-align: top; }
.lesson-table th { background: #0d6f9d; color: #fff; }
.lesson-table th:first-child { border-radius: .65rem 0 0 0; }
.lesson-table th:last-child { border-radius: 0 .65rem 0 0; }
.lesson-table tbody tr:nth-child(even) { background: #f4f9fc; }
.lesson-table tbody tr:hover { background: #e8f6fc; }

.code-panel, .output-panel { background: #101820; border: 1px solid #26394a; border-radius: .9rem; color: #dfe9f1; min-width: 0; overflow: hidden; }
.code-panel header, .output-panel header { align-items: center; background: #172634; border-bottom: 1px solid #2b4256; display: flex; justify-content: space-between; padding: .72rem .9rem; }
.code-file { align-items: center; display: flex; font-family: Consolas, "Courier New", monospace; gap: .45rem; }
.status-dot { background: #39d353; border-radius: 50%; display: inline-block; height: .58rem; margin-right: .5rem; width: .58rem; }
.copy-code { background: #22394c; border: 1px solid #496075; border-radius: .45rem; color: #e7f0f6; padding: .35rem .65rem; }
.copy-code:hover { background: #2d4b63; }
.code-panel pre, .output-panel pre { margin: 0; overflow: auto; padding: 1rem 0; }
.code-panel code, .output-panel code { color: inherit; display: block; font-family: Consolas, "Cascadia Code", "Courier New", monospace; font-size: .92em; line-height: 1.65; min-width: max-content; tab-size: 4; }
.code-line { counter-increment: code-line; display: block; min-height: 1.65em; padding: 0 1.1rem 0 4rem; position: relative; white-space: pre; }
.code-panel code { counter-reset: code-line; }
.code-line::before { color: #63788b; content: counter(code-line); left: 0; padding-right: .85rem; position: absolute; text-align: right; user-select: none; width: 3rem; }
.tok-keyword { color: #ff7b72; }
.tok-string { color: #a5d6ff; }
.tok-comment { color: #8b949e; font-style: italic; }
.tok-number { color: #79c0ff; }
.code-output-grid { display: grid; gap: 1rem; grid-template-columns: minmax(0, 1.35fr) minmax(260px, .65fr); }
.output-panel code { padding: 0 1.1rem; white-space: pre; }

.hierarchy-explorer { border: 1px solid #c6dce8; border-radius: 1rem; overflow: hidden; }
.hierarchy-toolbar { align-items: center; background: #eef8fc; border-bottom: 1px solid #c6dce8; display: flex; gap: 1rem; justify-content: space-between; padding: 1rem; }
.hierarchy-toolbar h3, .hierarchy-toolbar p { margin: 0; }
.hierarchy-actions { display: flex; gap: .4rem; }
.hierarchy-viewport { background: #fbfdff; max-height: 720px; overflow: auto; padding: 1rem; }
.hierarchy-canvas { display: grid; gap: 1rem; grid-template-columns: minmax(920px, 2fr) minmax(720px, 1.55fr) minmax(230px, .55fr); min-width: 1930px; transform-origin: top left; transition: transform .2s ease; }
.hierarchy-column { background: #f8fcff; border: 2px solid #3c82b2; border-radius: .8rem; padding: .9rem; }
.hierarchy-column > h4 { color: #092a4f; font-weight: 800; text-transform: uppercase; }
.hierarchy-branches { align-items: start; display: flex; gap: .65rem; }
.hierarchy-branch { flex: 1; }
.hierarchy-node { border: 2px solid #2460ac; border-radius: .55rem; color: #102746; margin: .55rem 0; padding: .55rem .65rem; text-align: center; }
.hierarchy-node strong { display: block; font-family: Consolas, monospace; }
.hierarchy-node small { display: block; font-size: .7rem; margin-top: .15rem; }
.hierarchy-relation-grid { display: grid; gap: .65rem; grid-template-columns: repeat(3, minmax(0, 1fr)); }
.hierarchy-relation { align-items: stretch; display: grid; grid-template-columns: minmax(0, 1fr) 34px minmax(0, 1fr); }
.hierarchy-relation .hierarchy-node { align-content: center; display: grid; margin: 0; min-height: 76px; }
.relation-connector { align-items: center; display: flex; justify-content: center; position: relative; }
.relation-connector::before { border-top: 3px solid #25384b; content: ""; left: 1px; position: absolute; right: 5px; top: 50%; }
.relation-connector::after { border-bottom: 6px solid transparent; border-left: 8px solid #25384b; border-top: 6px solid transparent; content: ""; position: absolute; right: 0; top: calc(50% - 5px); }
.relation-connector span { background: #fbfdff; color: #42566a; font-size: .58rem; font-weight: 800; padding: 0 .12rem; position: relative; text-transform: uppercase; transform: rotate(-90deg); z-index: 1; }
.relation-implements::before { border-top-style: dashed; }
.node-interface { background: #d6ebff; border-color: #2460ac; }
.node-abstract { background: #fff0c7; border-color: #e08c00; }
.node-concrete { background: #daf4dc; border-color: #2c9a49; }
.node-legacy { background: #e9dcfa; border-color: #8050bd; }
.node-concurrent { background: #d2f5f4; border-color: #138f91; }
.connector { color: #506275; font-size: .72rem; font-weight: 700; text-align: center; text-transform: uppercase; }
.hierarchy-legend { background: #fff; border-top: 1px solid #c6dce8; display: flex; flex-wrap: wrap; gap: .6rem; padding: .8rem 1rem; }
.hierarchy-legend span { border: 1px solid #c6d2dc; border-radius: .35rem; font-size: .74rem; padding: .25rem .5rem; }

.lesson-step-navigation { align-items: center; display: flex; gap: 1rem; justify-content: space-between; margin-top: 2rem; }
.is-explain-mode .material-top { display: none; }
.is-explain-mode .material-shell { max-width: none; padding: 1rem; }
.is-explain-mode .lesson-hero { background: transparent; border: 0; box-shadow: none; margin: 0 0 1rem; padding: 0; }
.is-explain-mode .lesson-hero > :not(.lesson-toolbar) { display: none; }
.is-explain-mode .lesson-layout { grid-template-columns: 250px minmax(0, 1fr); }
.is-explain-mode .step-nav { max-height: calc(100vh - 2rem); top: 1rem; }
.is-explain-mode .lesson-step { min-height: calc(100vh - 2rem); }

@media (max-width: 980px) {
  .lesson-layout { grid-template-columns: 1fr; }
  .step-nav { display: flex; gap: .35rem; overflow-x: auto; position: static; }
  .step-nav-title { display: none; }
  .step-nav a { flex: 0 0 auto; }
  .step-nav a span:last-child { display: none; }
  .code-output-grid { grid-template-columns: 1fr; }
  .is-explain-mode .lesson-layout { grid-template-columns: 1fr; }
}
@media (max-width: 620px) {
  .material-top { padding: .65rem .8rem; }
  .material-top-links a:first-child { display: none; }
  .material-shell { padding: .85rem; }
  .lesson-toolbar { align-items: stretch; }
  .lesson-tools { width: 100%; }
  .lesson-tools button { flex: 1; }
  .lesson-tools button span { display: none; }
  .lesson-step { border-radius: .9rem; padding: 1rem; }
  .hierarchy-toolbar { align-items: flex-start; flex-direction: column; }
}
@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
  *, *::before, *::after { animation-duration: .01ms !important; animation-iteration-count: 1 !important; transition-duration: .01ms !important; }
}
@media print {
  .material-top, .lesson-toolbar, .step-nav, .copy-code, .hierarchy-actions, .lesson-step-navigation { display: none !important; }
  body { background: #fff; color: #000; font-size: 11pt; }
  .material-shell { max-width: none; padding: 0; }
  .lesson-hero { box-shadow: none; border: 0; padding: 0; }
  .lesson-layout { display: block; }
  .lesson-step, .js .lesson-step[hidden] { box-shadow: none; border: 0; display: block !important; break-inside: avoid; padding: 1rem 0; }
  .code-panel, .output-panel { break-inside: avoid; }
}