:root {
  --bg: #fbfaf7;
  --bg-soft: #f3f1ec;
  --card: #ffffff;
  --ink: #1d1c22;
  --ink-soft: #55535e;
  --ink-faint: #8a8794;
  --line: #e4e1da;
  --accent: #4f46e5;
  --accent-soft: #eef0ff;
  --accent-ink: #3730a3;
  --good: #0f8a5f;
  --code-bg: #f6f5f1;
  --out-bg: #ffffff;
  --shadow: 0 1px 2px rgba(20, 18, 30, .05), 0 8px 24px rgba(20, 18, 30, .06);
  --radius: 14px;
  --font: "Inter", system-ui, -apple-system, "Segoe UI", sans-serif;
  --serif: "Fraunces", Georgia, serif;
  --mono: "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, monospace;
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #111116; --bg-soft: #17171e; --card: #1b1b23; --ink: #ecebf2; --ink-soft: #b2b0bd; --ink-faint: #85838f;
    --line: #2c2b36; --accent: #8b85ff; --accent-soft: #23224a; --accent-ink: #c4c1ff; --good: #3ccf91;
    --code-bg: #16161d; --out-bg: #f7f7f7; --shadow: 0 1px 2px rgba(0, 0, 0, .3), 0 8px 24px rgba(0, 0, 0, .25);
    color-scheme: dark;
  }
}
:root[data-theme="dark"] {
  --bg: #111116; --bg-soft: #17171e; --card: #1b1b23; --ink: #ecebf2; --ink-soft: #b2b0bd; --ink-faint: #85838f;
  --line: #2c2b36; --accent: #8b85ff; --accent-soft: #23224a; --accent-ink: #c4c1ff; --good: #3ccf91;
  --code-bg: #16161d; --out-bg: #f7f7f7; --shadow: 0 1px 2px rgba(0, 0, 0, .3), 0 8px 24px rgba(0, 0, 0, .25);
  color-scheme: dark;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; scroll-padding-top: 76px; -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 16px/1.65 var(--font); overflow-x: hidden; }
a { color: var(--accent); }
img { max-width: 100%; height: auto; }

/* top bar */
.topbar {
  position: sticky; top: 0; z-index: 20; display: flex; align-items: center; justify-content: space-between; gap: 12px;
  padding: 12px clamp(16px, 4vw, 40px); background: color-mix(in srgb, var(--bg) 88%, transparent);
  backdrop-filter: blur(10px); border-bottom: 1px solid var(--line);
}
.brand { display: flex; align-items: center; gap: 10px; color: var(--ink); text-decoration: none; font-weight: 600; letter-spacing: -.01em; white-space: nowrap; }
.brand-mark { color: var(--accent); display: flex; }
.brand b { color: var(--accent); }
.topnav { display: flex; align-items: center; gap: clamp(10px, 2.4vw, 26px); font-size: 15px; }
.topnav a { color: var(--ink-soft); text-decoration: none; }
.topnav a:hover { color: var(--accent); }
.topnav .lang-switch { border: 1px solid var(--line); border-radius: 999px; padding: 3px 11px; font-size: 13.5px; color: var(--ink); background: var(--card); white-space: nowrap; }
.topnav .lang-switch:hover { border-color: var(--accent); }
.theme-toggle { border: 1px solid var(--line); background: var(--card); color: var(--ink-soft); border-radius: 9px; width: 34px; height: 34px; display: grid; place-items: center; cursor: pointer; }
.theme-toggle .i-moon { display: none; }
:root[data-theme="dark"] .theme-toggle .i-sun { display: none; }
:root[data-theme="dark"] .theme-toggle .i-moon { display: block; }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) .theme-toggle .i-sun { display: none; }
  :root:not([data-theme="light"]) .theme-toggle .i-moon { display: block; }
}

/* buttons, chips */
.btn { display: inline-flex; align-items: center; gap: 6px; padding: 12px 20px; border-radius: 11px; font-weight: 600; text-decoration: none; border: 1px solid var(--line); color: var(--ink); background: var(--card); cursor: pointer; font: 600 15px var(--font); }
.btn:hover { border-color: var(--accent); color: var(--accent); }
.btn-primary { background: var(--accent); color: #fff; border-color: var(--accent); }
.btn-primary:hover { color: #fff; filter: brightness(1.08); }
.btn-ghost { background: transparent; }
.btn-small { padding: 7px 12px; font-size: 13.5px; border-radius: 9px; }
.btn-quiet { background: transparent; }
.chips { list-style: none; padding: 0; margin: 12px 0 0; display: flex; flex-wrap: wrap; gap: 6px; }
.chips li { font-size: 12.5px; padding: 3px 9px; border-radius: 999px; background: var(--accent-soft); color: var(--accent-ink); }
.eyebrow { text-transform: uppercase; letter-spacing: .08em; font-size: 12.5px; font-weight: 600; color: var(--accent); margin: 0 0 10px; }

/* landing */
.landing { max-width: 1180px; margin: 0 auto; padding: 0 clamp(16px, 4vw, 40px); }
.hero { display: grid; grid-template-columns: 1.15fr .85fr; gap: 40px; align-items: center; padding: clamp(40px, 8vw, 96px) 0 48px; }
.hero h1 { font-family: var(--serif); font-size: clamp(42px, 7vw, 76px); line-height: 1.02; letter-spacing: -.02em; margin: 0 0 18px; }
.hero h1 span { color: var(--accent); }
.lead { font-size: clamp(17px, 2vw, 19.5px); color: var(--ink-soft); max-width: 640px; margin: 0 0 26px; }
.cta { display: flex; flex-wrap: wrap; gap: 12px; }
.hero-stats { list-style: none; padding: 0; margin: 34px 0 0; display: grid; grid-template-columns: repeat(4, auto); justify-content: start; gap: 12px 34px; }
.hero-stats b { display: block; font-size: 26px; font-family: var(--serif); }
.hero-stats span { font-size: 13.5px; color: var(--ink-faint); }
.resume { margin-top: 22px; font-size: 15px; color: var(--ink-soft); }
.hero-art { background: radial-gradient(circle at 60% 40%, var(--accent-soft), transparent 70%); border-radius: 24px; }
.net { width: 100%; height: auto; display: block; }
.net line { stroke: var(--accent); stroke-opacity: .25; stroke-width: 1.4; }
.net line.pulse { stroke-opacity: .9; stroke-width: 2.2; transition: stroke-opacity .5s; }
.net circle { fill: var(--card); stroke: var(--accent); stroke-width: 2.2; }
.net circle.on { fill: var(--accent); }
.pillars { display: grid; grid-template-columns: repeat(3, 1fr); gap: 18px; padding: 8px 0 56px; }
.pillars div { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 22px; }
.pillars h3 { margin: 0 0 6px; font-size: 17px; }
.pillars p { margin: 0; color: var(--ink-soft); font-size: 15px; }
.section-head h2, .howto h2 { font-family: var(--serif); font-size: clamp(30px, 4vw, 40px); margin: 0 0 8px; letter-spacing: -.01em; }
.section-head p { color: var(--ink-soft); max-width: 720px; margin: 0; }
.progress-wrap { display: flex; align-items: center; gap: 12px; margin: 18px 0 6px; max-width: 420px; }
.progress { flex: 1; height: 8px; background: var(--bg-soft); border: 1px solid var(--line); border-radius: 99px; overflow: hidden; }
.progress-bar { display: block; height: 100%; width: 0; background: var(--good); transition: width .4s; }
.progress-label { font-size: 13.5px; color: var(--ink-faint); white-space: nowrap; }
.track { margin: 36px 0 12px; }
.track-head { display: flex; align-items: baseline; flex-wrap: wrap; gap: 4px 14px; margin-bottom: 14px; }
.track-head h2 { margin: 0; font-size: 22px; }
.track-head p { margin: 0; color: var(--ink-faint); font-size: 15px; flex-basis: 100%; }
.track-label { font-size: 12.5px; font-weight: 600; color: var(--accent); text-transform: uppercase; letter-spacing: .08em; }
.module-grid { display: grid; grid-template-columns: repeat(3, 1fr); gap: 16px; }
.module-card { position: relative; display: flex; flex-direction: column; background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 20px; text-decoration: none; color: var(--ink); box-shadow: var(--shadow); transition: transform .15s, border-color .15s; }
.module-card:hover { transform: translateY(-2px); border-color: var(--accent); }
.module-top { display: flex; justify-content: space-between; align-items: center; }
.module-num { font-family: var(--serif); font-size: 30px; color: var(--accent); line-height: 1; }
.module-done { display: none; font-size: 12.5px; font-weight: 600; color: var(--good); }
.module-card.is-done .module-done { display: inline; }
.module-card.is-done { border-color: color-mix(in srgb, var(--good) 45%, var(--line)); }
.module-card h3 { margin: 12px 0 6px; font-size: 18px; line-height: 1.3; }
.module-card p { margin: 0; color: var(--ink-soft); font-size: 14.5px; flex: 1; }
.module-meta { display: flex; flex-wrap: wrap; gap: 4px 14px; margin-top: 14px; font-size: 12.5px; color: var(--ink-faint); }
.howto { display: grid; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); gap: 40px; padding: 64px 0 40px; }
.howto p { color: var(--ink-soft); }
.shell { background: #16161d; color: #e6e6ef; padding: 16px 18px; border-radius: 12px; overflow-x: auto; font: 13.5px/1.7 var(--mono); }
.prereq { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); padding: 24px; }
.prereq h3 { margin: 0 0 8px; font-size: 17px; }
.prereq h3 + ul { margin-bottom: 20px; }
.prereq ul { padding-left: 18px; margin: 0; color: var(--ink-soft); font-size: 14.5px; }
.prereq li { margin: 6px 0; }
.reading { font-size: 13.5px !important; }

/* lesson */
.lesson-layout { display: grid; grid-template-columns: 250px minmax(0, 1fr) 220px; gap: 36px; max-width: 1440px; margin: 0 auto; padding: 0 clamp(16px, 3vw, 32px); }
.sidebar, .toc { position: sticky; top: 74px; align-self: start; max-height: calc(100vh - 90px); overflow-y: auto; padding: 24px 0; }
.side-toggle { display: none; }
.side-home { display: block; font-size: 14px; text-decoration: none; margin-bottom: 12px; }
.progress.small { height: 6px; margin-bottom: 14px; }
.side-list { list-style: none; padding: 0; margin: 0; }
.side-list a { display: flex; gap: 10px; padding: 8px 10px; border-radius: 9px; text-decoration: none; color: var(--ink-soft); font-size: 14px; line-height: 1.35; }
.side-list a:hover { background: var(--bg-soft); color: var(--ink); }
.side-list a.current { background: var(--accent-soft); color: var(--accent-ink); font-weight: 600; }
.side-num { font-family: var(--mono); font-size: 12.5px; color: var(--ink-faint); padding-top: 1px; }
.side-list a.is-done .side-num { color: var(--good); }
.side-list a.is-done .side-num::after { content: " ✓"; }
.toc p { font-size: 12.5px; text-transform: uppercase; letter-spacing: .08em; color: var(--ink-faint); margin: 0 0 8px; font-weight: 600; }
.toc ol { list-style: none; padding: 0; margin: 0; border-left: 1px solid var(--line); }
.toc a { display: block; padding: 4px 0 4px 12px; margin-left: -1px; border-left: 2px solid transparent; font-size: 13.5px; color: var(--ink-faint); text-decoration: none; line-height: 1.4; }
.toc a:hover { color: var(--ink); }
.toc a.active { color: var(--accent); border-left-color: var(--accent); }
.lesson { min-width: 0; padding: 32px 0 60px; }
.lesson-head { border-bottom: 1px solid var(--line); padding-bottom: 22px; margin-bottom: 8px; }
.lesson-head h1 { font-family: var(--serif); font-size: clamp(32px, 4.5vw, 46px); line-height: 1.1; margin: 0 0 12px; letter-spacing: -.015em; }
.lesson-head .lead { margin-bottom: 0; }
.lesson-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 8px 10px; margin-top: 18px; }
.lesson-actions .meta { font-size: 13.5px; color: var(--ink-faint); margin-right: 6px; }

/* rendered notebook */
.notebook { font-size: 16.5px; }
.notebook .prompt { display: none; }
.notebook .cell { margin: 0; }
.notebook .text_cell_render { max-width: 760px; }
.notebook h1 { font-family: var(--serif); font-size: 30px; margin: 48px 0 12px; padding-top: 18px; border-top: 1px solid var(--line); }
.notebook h2 { font-size: 24px; margin: 44px 0 10px; letter-spacing: -.01em; }
.notebook h3 { font-size: 19px; margin: 30px 0 8px; }
.notebook p, .notebook li { color: var(--ink); }
.notebook hr { border: 0; border-top: 1px solid var(--line); margin: 36px 0; }
.notebook blockquote { margin: 16px 0; padding: 4px 18px; border-left: 3px solid var(--accent); color: var(--ink-soft); background: var(--bg-soft); border-radius: 0 10px 10px 0; }
.notebook table { border-collapse: collapse; margin: 18px 0; font-size: 14.5px; display: block; overflow-x: auto; max-width: 100%; }
.notebook th, .notebook td { border: 1px solid var(--line); padding: 7px 11px; text-align: left; vertical-align: top; }
.notebook th { background: var(--bg-soft); font-weight: 600; }
.notebook code { font-family: var(--mono); font-size: .86em; background: var(--bg-soft); padding: 1px 5px; border-radius: 5px; }
.notebook .text_cell_render pre { background: var(--code-bg); border: 1px solid var(--line); border-radius: 10px; padding: 14px; overflow-x: auto; font: 13px/1.45 var(--mono); }
.notebook .text_cell_render pre code { background: none; padding: 0; font-size: inherit; }
.notebook .input_area { position: relative; margin: 18px 0 0; border: 1px solid var(--line); border-radius: 12px 12px 0 0; background: var(--code-bg); }
.notebook .input_area::before { content: "Python"; position: absolute; top: 7px; right: 12px; font: 600 11px var(--mono); color: var(--ink-faint); letter-spacing: .05em; }
.notebook .code_cell .input_area:last-child { border-radius: 12px; }
.notebook .highlight { background: transparent !important; }
.notebook .highlight pre { margin: 0; padding: 16px 18px; overflow-x: auto; font: 13.4px/1.6 var(--mono); }
.notebook .output_wrapper { border: 1px solid var(--line); border-top: 0; border-radius: 0 0 12px 12px; padding: 12px 16px; background: var(--card); margin-bottom: 8px; }
.notebook .output_text pre { margin: 4px 0; font: 13px/1.5 var(--mono); white-space: pre-wrap; word-break: break-word; color: var(--ink-soft); }
.notebook .output_png { overflow-x: auto; }
.notebook .output_png img { display: block; margin: 8px auto; background: var(--out-bg); border-radius: 8px; padding: 4px; max-width: 100%; }
.notebook .output_stderr { display: none; }
.notebook mjx-container[display="true"] { overflow-x: auto; overflow-y: hidden; max-width: 100%; }
.complete-box { margin: 48px 0 24px; text-align: center; }
.complete-btn.is-done { background: var(--good); border-color: var(--good); }
.pager { display: grid; grid-template-columns: 1fr 1fr; gap: 14px; }
.pager a { display: flex; flex-direction: column; gap: 2px; padding: 16px 18px; border: 1px solid var(--line); border-radius: 12px; text-decoration: none; background: var(--card); color: var(--ink); }
.pager a:hover { border-color: var(--accent); }
.pager small { color: var(--ink-faint); font-size: 12.5px; }
.pager .next { text-align: right; }

.footer { border-top: 1px solid var(--line); padding: 26px clamp(16px, 4vw, 40px); text-align: center; color: var(--ink-faint); font-size: 13.5px; }
.footer p { margin: 4px 0; }

@media (max-width: 1180px) {
  .lesson-layout { grid-template-columns: 230px minmax(0, 1fr); }
  .toc { display: none; }
}
@media (max-width: 960px) {
  .hero { grid-template-columns: 1fr; padding-top: 36px; }
  .hero-art { max-width: 460px; }
  .module-grid { grid-template-columns: repeat(2, 1fr); }
  .pillars, .howto { grid-template-columns: minmax(0, 1fr); }
}
@media (max-width: 860px) {
  .lesson-layout { grid-template-columns: minmax(0, 1fr); gap: 0; }
  .sidebar { position: static; max-height: none; padding: 14px 0 0; }
  .side-toggle { display: block; width: 100%; padding: 10px 14px; border: 1px solid var(--line); border-radius: 10px; background: var(--card); color: var(--ink); font: 600 14px var(--font); text-align: left; cursor: pointer; }
  .side-inner { display: none; padding-top: 12px; }
  .sidebar.open .side-inner { display: block; }
  .lesson { padding-top: 20px; }
}
@media (max-width: 620px) {
  .module-grid { grid-template-columns: minmax(0, 1fr); }
  .hero-stats { grid-template-columns: repeat(2, auto); }
  .topnav { gap: 10px; font-size: 14px; }
  .brand span:last-child { font-size: 15px; }
  .pager { grid-template-columns: 1fr; }
  .pager .next { text-align: left; }
  .notebook { font-size: 16px; }
}
@media (max-width: 460px) {
  .topnav { gap: 8px; font-size: 13px; }
  .topnav .lang-switch { padding: 2px 8px; font-size: 12.5px; }
}
@media (max-width: 400px) {
  .brand span:last-child { display: none; }
}

/* Python 1001: code card in the hero instead of the animated network */
.hero-code { margin: 0; padding: 1.4rem 1.5rem; border-radius: 16px; background: #0f172a; color: #e2e8f0;
  font: 500 .95rem/1.6 "JetBrains Mono", monospace; box-shadow: 0 20px 50px rgba(15, 23, 42, .25); overflow-x: auto; }
.hero-code .k { color: #c084fc; } .hero-code .s { color: #86efac; } .hero-code .c { color: #94a3b8; font-style: italic; }
@media (max-width: 800px) { .hero-code { font-size: .8rem; } }