:root {
  --bg:#0d1117; --surface:#161b22; --border:#30363d; --text:#e6edf3;
  --text-muted:#8b949e; --accent:#58a6ff; --accent-dim:#1f6feb22;
  --line-highlight:#1f6feb33; --comment-block:#f0883e18;
  --chapter-accent:#f0883e; --chapter-dim:#f0883e18;
}
*{margin:0;padding:0;box-sizing:border-box}
body{font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif;background:var(--bg);color:var(--text);line-height:1.6}
a{color:var(--accent);text-decoration:none}
a:hover{text-decoration:underline}
code{background:var(--surface);border:1px solid var(--border);border-radius:4px;padding:1px 5px;font-size:.85em}
.github-corner{position:absolute;top:0;right:0;z-index:20;color:var(--bg)}
.github-corner svg{fill:var(--accent);color:var(--bg);border:0}
.github-corner:hover .octo-arm{animation:octocat-wave 560ms ease-in-out}
@keyframes octocat-wave{0%,100%{transform:rotate(0)}20%,60%{transform:rotate(-25deg)}40%,80%{transform:rotate(10deg)}}
@media (max-width:500px){.github-corner:hover .octo-arm{animation:none}.github-corner .octo-arm{animation:octocat-wave 560ms ease-in-out}}
.container{max-width:860px;margin:0 auto;padding:60px 24px}
.back-link{color:var(--text-muted);text-decoration:none;font-size:.9rem}
.back-link:hover{color:var(--accent)}

/* --- Index page --- */
.site-title{font-size:2.4rem;font-weight:700;letter-spacing:-.03em;margin-bottom:8px}
.site-tagline{font-size:1.15rem;color:var(--text-muted);margin-bottom:16px;max-width:600px}
.search-input{width:100%;max-width:400px;padding:8px 14px;border:1px solid var(--border);border-radius:8px;background:var(--surface);color:var(--text);font-size:.95rem;margin-bottom:32px;outline:none;transition:border-color .15s}
.search-input:focus{border-color:var(--accent)}
.search-input::placeholder{color:var(--text-muted)}
.lab-match-context{font-size:.78rem;color:var(--text-muted);margin-top:2px;line-height:1.4}
.lab-match-context mark{background:#f0883e44;color:var(--text);border-radius:2px;padding:0 1px}
.chapter{margin-bottom:32px}
.chapter-header{display:flex;align-items:center;gap:12px;padding:14px 20px;border:1px solid var(--border);border-radius:8px;cursor:pointer;transition:border-color .15s,background .15s;user-select:none}
.chapter-header:hover{border-color:var(--chapter-accent);background:var(--chapter-dim)}
.chapter-header.open{border-bottom-left-radius:0;border-bottom-right-radius:0;border-color:var(--chapter-accent);background:var(--chapter-dim)}
.chapter-num{font-size:.75rem;font-weight:700;color:var(--chapter-accent);text-transform:uppercase;letter-spacing:.05em;min-width:24px}
.chapter-title{font-size:1.1rem;font-weight:700;flex:1}
.chapter-count{font-size:.8rem;color:var(--text-muted)}
.chapter-arrow{color:var(--text-muted);font-size:.8rem;transition:transform .2s}
.chapter-header.open .chapter-arrow{transform:rotate(90deg)}
.chapter-link{font-size:.78rem;color:var(--chapter-accent);text-decoration:none;padding:2px 8px;border-radius:4px;transition:background .15s}
.chapter-link:hover{background:var(--chapter-dim);text-decoration:underline}
.chapter-labs{list-style:none;border:1px solid var(--border);border-top:none;border-radius:0 0 8px 8px;overflow:hidden;display:none}
.chapter-labs.open{display:block}
.lab-list{list-style:none}
.lab-item{border:1px solid var(--border);border-radius:8px;margin-bottom:8px;transition:border-color .15s,background .15s}
.lab-item:hover{border-color:var(--accent);background:var(--accent-dim)}
.chapter-labs .lab-item{border-radius:0;margin-bottom:0;border-left:none;border-right:none;border-top:none}
.lab-item a{display:flex;align-items:flex-start;gap:16px;padding:14px 20px;text-decoration:none;color:inherit}
.lab-num{font-size:.85rem;font-weight:600;color:var(--text-muted);font-variant-numeric:tabular-nums;min-width:28px;padding-top:2px}
.lab-title-text{font-size:1.05rem;font-weight:600;color:var(--accent)}
.lab-tagline{font-size:.88rem;color:var(--text-muted);margin-top:4px;line-height:1.45}
.lab-progress{display:flex;align-items:center;gap:6px;margin-top:4px}
.lab-progress-bar{width:60px;height:4px;background:var(--border);border-radius:2px;overflow:hidden}
.lab-progress-fill{height:100%;background:var(--accent);border-radius:2px;transition:width .3s}
.lab-progress span{font-size:.72rem;color:var(--text-muted)}

/* --- Chapter page --- */
.ch-num{font-size:.85rem;font-weight:700;color:var(--chapter-accent);text-transform:uppercase;letter-spacing:.05em;margin-top:24px;display:block}
.ch-title{font-size:2rem;font-weight:700;letter-spacing:-.03em;margin:4px 0 16px}
.ch-desc{font-size:1rem;line-height:1.7;margin-bottom:24px}
.ch-desc p{margin-bottom:12px}
.diagram-box{border:1px solid var(--border);border-radius:8px;padding:16px;background:var(--surface);margin-bottom:32px;text-align:center;overflow-x:auto}
.diagram-box svg{height:auto;display:inline-block}
.nav-row{display:flex;gap:12px;margin-top:32px}
.nav-btn{padding:8px 16px;border:1px solid var(--border);border-radius:6px;text-decoration:none;color:var(--text-muted);font-size:.9rem;transition:all .15s}
.nav-btn:hover{border-color:var(--accent);color:var(--accent)}

/* --- Lab page --- */
.lab-header{padding:16px 90px 16px 24px;border-bottom:1px solid var(--border);flex-shrink:0}
.lab-header-top{display:flex;align-items:center;gap:12px}
.lab-header h1{font-size:1.3rem;font-weight:700;letter-spacing:-.02em;flex:1;min-width:0}
.lab-header .tagline{color:var(--text-muted);font-size:.85rem;line-height:1.45;margin-top:6px}
.progress-bar{height:4px;background:var(--border);border-radius:2px;margin-top:8px;position:relative}
.progress-fill{height:100%;background:#58a6ff;border-radius:2px;transition:width 0.3s}
.progress-text{font-size:0.75rem;color:var(--text-muted);position:absolute;right:0;top:6px}
.code-line.visited .line-num{border-left:2px solid #58a6ff33}
.lab-main{display:flex;flex:1;overflow:hidden}
.code-panel{flex:1;overflow-y:auto;min-width:200px;position:relative}
.h-resize{width:5px;min-width:5px;cursor:ew-resize;background:var(--border);transition:background .15s;flex-shrink:0}
.h-resize:hover,.h-resize.dragging{background:var(--accent)}
.code-table{width:100%;border-collapse:collapse}
.code-table td{padding:0;vertical-align:top;white-space:pre;font-family:"SF Mono","Fira Code","JetBrains Mono",Consolas,monospace;font-size:13px;line-height:1.65}
.line-num{width:48px;min-width:48px;text-align:right;padding-right:12px!important;color:var(--text-muted);opacity:.5;user-select:none;cursor:pointer}
.line-content{padding-left:8px!important;padding-right:16px!important}
.code-line{cursor:pointer;transition:background .1s}
.code-line:hover{background:var(--accent-dim)}
.code-line.selected .line-num{opacity:1;color:var(--accent)}
.code-line.selected{background:var(--line-highlight)}
.code-line.context{background:var(--comment-block)}
.code-line.stale .line-num{color:#d29922}
.code-coach{position:absolute;top:12px;right:16px;display:flex;align-items:flex-start;gap:12px;max-width:320px;padding:10px 12px;border:1px solid var(--border);border-radius:10px;background:color-mix(in srgb, var(--surface) 88%, var(--bg));box-shadow:0 8px 24px rgba(1,4,9,.28);z-index:5}
.code-coach.hidden{display:none}
.code-coach-copy{min-width:0}
.code-coach-title{font-size:.82rem;font-weight:600;color:var(--text)}
.code-coach-text{font-size:.76rem;line-height:1.45;color:var(--text-muted);margin-top:2px}
.code-coach-close{background:none;border:none;color:var(--text-muted);font-size:1rem;line-height:1;cursor:pointer;padding:2px;border-radius:4px;flex-shrink:0}
.code-coach-close:hover{color:var(--text);background:var(--accent-dim)}
.explain-search{padding:0 0 12px 0;border-bottom:1px solid var(--border);margin-bottom:12px}
.explain-search .search-input{width:100%;padding:6px 10px;border:1px solid var(--border);border-radius:6px;background:var(--bg);color:var(--text);font-size:.85rem}
.explain-search .search-input:focus{border-color:var(--accent);outline:none}
.search-results{max-height:200px;overflow-y:auto;margin-top:8px}
.search-result{padding:6px 8px;border-radius:4px;cursor:pointer;font-size:.82rem;color:var(--text-muted);line-height:1.4}
.search-result:hover{background:var(--accent-dim);color:var(--text)}
.search-result .line-ref{color:var(--accent);font-weight:600;margin-right:4px}
.search-result mark{background:#f0883e44;color:var(--text);border-radius:2px;padding:0 1px}
.search-count{font-size:.75rem;color:var(--text-muted);margin-top:4px}
.explain-panel{width:35%;min-width:200px;max-width:70%;overflow-y:auto;padding:24px;display:flex;flex-direction:column;flex-shrink:0}
.explain-text{font-size:.92rem;line-height:1.65}
.explain-text code{background:var(--surface);border:1px solid var(--border);border-radius:4px;padding:1px 5px;font-size:.85em}
.explain-line-ref{font-size:.8rem;color:var(--text-muted);margin-bottom:8px;font-family:monospace;display:flex;align-items:center;gap:8px;flex-wrap:wrap}
.tips-btn{background:none;border:1px solid var(--border);color:var(--text-muted);padding:2px 8px;border-radius:999px;cursor:pointer;font-size:.72rem;line-height:1.2;transition:all .15s}
.tips-btn:hover{border-color:var(--accent);color:var(--accent);background:var(--accent-dim)}
.line-nav{display:inline-flex;align-items:center;gap:6px;padding:2px 8px;border:1px solid var(--border);border-radius:999px;background:var(--surface)}
.line-nav.hidden{display:none}
.line-nav-btn{width:24px;height:24px;display:inline-flex;align-items:center;justify-content:center;border:1px solid var(--border);border-radius:999px;background:transparent;color:var(--text-muted);font:inherit;cursor:pointer;transition:all .15s}
.line-nav-btn:hover:not(:disabled){border-color:var(--accent);color:var(--accent);background:var(--accent-dim)}
.line-nav-btn:disabled{opacity:.35;cursor:default}
.line-nav-hint{font-size:.72rem;color:var(--text-muted);letter-spacing:.01em}
.overview-btn{background:none;border:1px solid var(--border);color:var(--text-muted);padding:2px 8px;border-radius:4px;cursor:pointer;font-size:.75rem;transition:all .15s}
.overview-btn:hover{border-color:var(--accent);color:var(--accent)}
.lab-desc p{margin-bottom:10px}
.learning-objectives{margin-top:16px;padding:12px 16px;border:1px solid var(--border);border-radius:8px;background:var(--surface)}
.learning-objectives h3{font-size:.85rem;color:var(--text-muted);text-transform:uppercase;letter-spacing:.05em;margin-bottom:8px}
.learning-objectives ul{list-style:none;padding:0}
.learning-objectives li{font-size:.88rem;line-height:1.5;padding:4px 0 4px 20px;position:relative;color:var(--text)}
.learning-objectives li::before{content:"\2713";position:absolute;left:0;color:var(--accent);font-weight:700}
.exercises{margin-top:16px;padding:12px 16px;border:1px solid var(--border);border-radius:8px;background:var(--surface)}
.exercises h3{font-size:.85rem;color:var(--chapter-accent);text-transform:uppercase;letter-spacing:.05em;margin-bottom:8px}
.exercise{padding:8px 0;border-bottom:1px solid var(--border)}
.exercise:last-child{border-bottom:none}
.exercise-label{display:flex;align-items:flex-start;gap:8px;cursor:pointer}
.exercise-check{margin-top:3px;accent-color:var(--accent);cursor:pointer}
.exercise-check:checked+.exercise-prompt{text-decoration:line-through;opacity:.6}
.exercise-count{font-size:.75rem;color:var(--text-muted);margin-top:4px}
.exercise-prompt{font-size:.88rem;line-height:1.5}
.exercise-hint{font-size:.82rem;color:var(--text-muted);margin-top:4px;cursor:pointer}
.reset-progress{margin-top:16px;padding-top:12px;border-top:1px solid var(--border);display:flex;gap:16px;flex-wrap:wrap}
.reset-progress-btn{background:none;border:none;color:var(--text-muted);font-size:.78rem;cursor:pointer;padding:0;text-decoration:underline;opacity:.6;transition:opacity .15s}
.reset-progress-btn:hover{opacity:1;color:var(--text)}
.exercise-hint-text{display:none;margin-top:4px;padding:8px;background:var(--bg);border-radius:4px;font-size:.82rem}
.stale-warning{display:inline-flex;align-items:center;gap:6px;background:#d2992233;border:1px solid #d2992255;border-radius:6px;padding:4px 10px;margin-top:12px;font-size:.8rem;color:#d29922}
.stale-dot{width:8px;height:8px;background:#d29922;border-radius:50%;flex-shrink:0}
.diagram-container{margin-top:16px;border:1px solid var(--border);border-radius:8px;padding:12px;background:var(--surface);overflow-x:auto;overflow-y:hidden;text-align:center}
.diagram-container svg{height:auto;display:inline-block}
.diagram-container.hidden{display:none}
.nav-footer{display:flex;gap:8px;margin-top:auto;padding-top:16px;border-top:1px solid var(--border);flex-wrap:wrap}
.nav-link{padding:6px 12px;border:1px solid var(--border);border-radius:6px;text-decoration:none;color:var(--text-muted);font-size:.82rem;transition:all .15s;white-space:nowrap}
.nav-link:hover{border-color:var(--accent);color:var(--accent)}
.nav-link.chapter{border-color:var(--chapter-accent);color:var(--chapter-accent);opacity:.7}
.nav-link.chapter:hover{opacity:1}
.edit-btn,.reset-btn,.play-btn,.stop-btn{display:none;align-items:center;gap:6px;padding:5px 14px;border-radius:6px;border:1px solid var(--border);background:transparent;color:var(--text);font-size:.82rem;cursor:pointer;transition:all .15s}
.play-btn,.stop-btn{margin-left:auto}
.edit-btn:hover{border-color:var(--accent);color:var(--accent)}
.edit-btn .icon{color:var(--accent)}
.reset-btn:hover{border-color:var(--chapter-accent);color:var(--chapter-accent)}
.reset-btn .icon{color:var(--chapter-accent)}
.code-panel.edit-mode{border-left:3px solid var(--accent)}
.editor-wrapper{display:none;width:100%;height:100%;overflow:hidden}
.editor-gutter{width:48px;min-width:48px;padding:8px 12px 8px 0;text-align:right;font-family:"SF Mono","Fira Code","JetBrains Mono",Consolas,monospace;font-size:13px;line-height:1.65;color:var(--text-muted);opacity:.5;background:var(--bg);overflow:hidden;white-space:pre;user-select:none;flex-shrink:0}
.editor-code-area{position:relative;flex:1;overflow:hidden}
.editor-highlight,.code-editor{position:absolute;top:0;left:0;width:100%;height:100%;margin:0;border:none;padding:8px 8px 8px 8px;font-family:"SF Mono","Fira Code","JetBrains Mono",Consolas,monospace;font-size:13px;line-height:1.65;tab-size:4;white-space:pre;overflow:auto}
.editor-highlight{background:var(--bg);color:var(--text);pointer-events:none;z-index:0}
.code-editor{background:transparent;color:transparent;caret-color:var(--text);z-index:1;resize:none;outline:none;-webkit-text-fill-color:transparent}
.run-hint{display:none;align-items:center;gap:4px;margin-left:auto;font-size:.75rem;color:var(--text-muted);border:1px dashed var(--border);border-radius:6px;padding:4px 10px;cursor:help}
.play-btn:hover{border-color:#3fb950;color:#3fb950}
.play-btn .icon{color:#3fb950}
.stop-btn:hover{border-color:#f85149;color:#f85149}
.stop-btn .icon{color:#f85149}
.terminal-panel{display:none;flex-direction:column;border-top:2px solid var(--border);background:#010409;flex-shrink:0;height:30vh;min-height:100px;max-height:70vh;position:relative}
.terminal-panel.open{display:flex}
.terminal-resize-handle{height:5px;cursor:ns-resize;background:transparent;flex-shrink:0;transition:background .15s}
.terminal-resize-handle:hover,.terminal-resize-handle.dragging{background:var(--accent)}
.terminal-header{display:flex;align-items:center;gap:10px;padding:8px 16px;background:#161b22;border-bottom:1px solid var(--border);font-size:.8rem;flex-shrink:0}
.terminal-title{font-weight:600;color:var(--text-muted);display:flex;align-items:center;gap:6px}
.terminal-status{color:var(--text-muted);font-family:monospace;font-size:.75rem}
.terminal-status.running{color:#3fb950}
.terminal-status.error{color:#f85149}
.terminal-close{margin-left:auto;background:none;border:none;color:var(--text-muted);cursor:pointer;font-size:1.1rem;padding:2px 6px;border-radius:4px}
.terminal-close:hover{background:var(--border);color:var(--text)}
.terminal-output{flex:1;overflow-y:auto;padding:12px 16px;font-family:"SF Mono","Fira Code","JetBrains Mono",Consolas,monospace;font-size:12px;line-height:1.55;color:#c9d1d9;white-space:pre-wrap;word-break:break-all}
.terminal-scroll-bottom{display:none;position:absolute;bottom:12px;right:20px;width:32px;height:32px;align-items:center;justify-content:center;border-radius:50%;border:1px solid var(--border);background:var(--surface);color:var(--accent);font-size:16px;cursor:pointer;z-index:10;box-shadow:0 2px 8px rgba(0,0,0,.4);transition:all .15s}
.terminal-scroll-bottom:hover{background:var(--accent);color:var(--bg);border-color:var(--accent)}
.terminal-output .cmd{color:#8b949e}
.terminal-spinner{display:inline-block;width:8px;height:8px;border:2px solid #3fb950;border-top-color:transparent;border-radius:50%;animation:spin .8s linear infinite}
@keyframes spin{to{transform:rotate(360deg)}}
.terminal-panel.fullscreen{position:fixed;top:0;left:0;right:0;bottom:0;height:100vh!important;max-height:100vh!important;z-index:100;border-top:none}
.explain-toggle{display:none}
@media(max-width:900px){.lab-main{flex-direction:column}.explain-panel{width:100%;max-width:100%;min-width:0;border-top:1px solid var(--border);max-height:40vh}.code-panel{border-right:none}.h-resize{display:none}.code-table td.line-content{white-space:pre-wrap;word-break:break-word}
.code-coach{left:12px;right:12px;max-width:none}
.line-nav-hint{display:none}
.explain-toggle{display:flex;align-items:center;justify-content:center;width:100%;padding:8px;background:var(--surface);border:none;border-top:1px solid var(--border);color:var(--accent);font-size:.82rem;cursor:pointer;flex-shrink:0}
.explain-toggle::before{content:"▼ ";font-size:.7rem}
.explain-toggle.collapsed::before{content:"▶ "}
.explain-panel.mobile-hidden{display:none}}
.error-banner{position:fixed;bottom:0;left:0;right:0;padding:8px 16px;background:#f8514922;border-top:1px solid #f85149;color:#f85149;font-size:.82rem;z-index:200;display:flex;align-items:center;gap:8px}
.error-banner button{background:none;border:1px solid #f85149;color:#f85149;padding:2px 8px;border-radius:4px;cursor:pointer;font-size:.75rem;margin-left:auto}

/* --- Knowledge checks --- */
.knowledge-checks{margin-top:32px}
.knowledge-checks h2{font-size:1.1rem;font-weight:600;margin-bottom:16px;color:var(--chapter-accent)}
.quiz-card{border:1px solid var(--border);border-radius:8px;padding:16px;margin-bottom:12px;background:var(--surface)}
.quiz-question{font-size:.95rem;font-weight:600;margin-bottom:12px}
.quiz-options{list-style:none;padding:0}
.quiz-option{display:flex;align-items:center;gap:8px;padding:6px 8px;border-radius:4px;cursor:pointer;font-size:.88rem;transition:background .15s}
.quiz-option:hover{background:var(--accent-dim)}
.quiz-option.correct{background:#3fb95022;border-left:3px solid #3fb950}
.quiz-option.incorrect{background:#f8514922;border-left:3px solid #f85149;opacity:.6}
.quiz-option.selected{font-weight:600}
.quiz-option.answered{pointer-events:none}
.quiz-explanation{margin-top:8px;padding:8px 12px;background:var(--bg);border-radius:4px;font-size:.85rem;color:var(--text-muted);display:none}
.quiz-explanation.visible{display:block}
.quiz-score{font-size:.85rem;color:var(--text-muted);margin-top:8px}