:root{
  --bg:#f6f7fb; --surface:#fff; --ink:#151a2d; --muted:#5b6478; --line:#e3e6ef;
  --brand:#1f4fd8; --brand-ink:#fff; --accent:#ffcf3f; --code:#f3f5f9; --radius:12px;
  --shadow:0 1px 2px rgba(16,24,40,.06),0 4px 14px rgba(16,24,40,.06);
}
@media (prefers-color-scheme:dark){
  :root{--bg:#0f1220;--surface:#171b2e;--ink:#e8eaf2;--muted:#a3abc2;--line:#2a3050;--brand:#6c8cff;--code:#11152a;
  --shadow:0 1px 2px rgba(0,0,0,.4)}
  .nb-body{color-scheme:light}
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0;background:var(--bg);color:var(--ink);font:16px/1.6 system-ui,-apple-system,"Segoe UI",Roboto,Arial,sans-serif;-webkit-font-smoothing:antialiased}
a{color:var(--brand);text-decoration:none} a:hover{text-decoration:underline}
img{max-width:100%}
.wrap{max-width:1180px;margin:0 auto;padding:0 16px}
h1{font-size:clamp(1.6rem,3.2vw,2.4rem);line-height:1.2;margin:.2em 0 .5em;letter-spacing:-.01em}
h2{font-size:1.3rem;line-height:1.3}
code{background:var(--code);padding:.1em .35em;border-radius:5px;font-size:.92em}

/* top bar */
.topbar{position:sticky;top:0;z-index:10;background:color-mix(in srgb,var(--surface) 92%,transparent);backdrop-filter:blur(8px);border-bottom:1px solid var(--line)}
.topbar-inner{display:flex;align-items:center;justify-content:space-between;height:60px;gap:12px}
.brand{display:flex;align-items:center;gap:10px;font-weight:700;color:var(--ink);white-space:nowrap}
.brand:hover{text-decoration:none}
.topbar nav{display:flex;gap:6px;align-items:center}
.topbar nav a{color:var(--muted);padding:6px 10px;border-radius:8px;font-size:.95rem}
.topbar nav a:hover{background:var(--bg);color:var(--ink);text-decoration:none}
.topbar nav a.yt{background:#e62117;color:#fff;font-weight:600}
.topbar nav a.yt:hover{background:#c41a12;color:#fff}

/* hero */
.hero{background:linear-gradient(135deg,#1f4fd8 0%,#3b2fb5 100%);color:#fff;padding:48px 0 40px}
.hero h1{color:#fff;margin-top:0}
.hero .lead{color:#dfe5ff;font-size:1.15rem;max-width:720px}
.stats{display:flex;gap:22px;flex-wrap:wrap;margin:18px 0 22px;color:#dfe5ff}
.stats b{color:var(--accent);font-size:1.3rem;margin-right:4px}
.search input{width:100%;max-width:640px;padding:14px 16px;border-radius:10px;border:0;font-size:1rem;box-shadow:0 6px 24px rgba(0,0,0,.18);background:#fff;color:#151a2d}
.lead{color:var(--muted);font-size:1.05rem}

.chips{display:flex;flex-wrap:wrap;gap:8px;margin:24px 0 8px}
.chip{background:var(--surface);border:1px solid var(--line);border-radius:999px;padding:5px 12px;color:var(--ink);font-size:.9rem}
.chip span{color:var(--muted);margin-left:4px}
.chip:hover{border-color:var(--brand);text-decoration:none}

.series{margin:30px 0 10px;scroll-margin-top:70px}
.series-head{display:flex;justify-content:space-between;align-items:baseline;gap:12px;flex-wrap:wrap}
.series-head h2{margin:0 0 12px}
.series-head h2 a{color:var(--ink)}
.more{font-weight:600;font-size:.95rem}

.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(250px,1fr));gap:16px}
.extra{display:contents}
.extra[hidden]{display:none}
.card{background:var(--surface);border:1px solid var(--line);border-radius:var(--radius);overflow:hidden;color:var(--ink);box-shadow:var(--shadow);transition:transform .15s,box-shadow .15s;display:flex;flex-direction:column}
.card:hover{transform:translateY(-2px);box-shadow:0 8px 24px rgba(16,24,40,.12);text-decoration:none}
.thumb{position:relative;aspect-ratio:16/9;background:#dde2ee}
.thumb img{width:100%;height:100%;object-fit:cover;display:block}
.play{position:absolute;inset:auto 10px 10px auto;background:rgba(0,0,0,.7);color:#fff;border-radius:6px;font-size:.75rem;padding:3px 8px}
.card-body{padding:12px 14px 14px}
.card h3{font-size:.98rem;line-height:1.35;margin:0 0 6px}
.card .num{display:inline-block;font-size:.75rem;font-weight:700;color:var(--brand);margin-bottom:2px}
.card .meta{margin:0;font-size:.8rem;color:var(--muted)}

/* lesson */
.lesson{max-width:980px;padding-top:18px;padding-bottom:40px}
.crumbs{font-size:.9rem;color:var(--muted);margin:0 0 4px}
.eyebrow{color:var(--brand);font-weight:700;font-size:.85rem;text-transform:uppercase;letter-spacing:.04em;margin:10px 0 0}
.video{position:relative;aspect-ratio:16/9;background:#000;border-radius:var(--radius);overflow:hidden;box-shadow:var(--shadow)}
.video iframe{position:absolute;inset:0;width:100%;height:100%;border:0}
.actions{display:flex;flex-wrap:wrap;gap:10px;margin:18px 0}
.btn{display:inline-flex;align-items:center;gap:8px;padding:10px 16px;border-radius:10px;font-weight:600;font-size:.95rem;border:1px solid var(--line);background:var(--surface);color:var(--ink)}
.btn:hover{text-decoration:none;border-color:var(--brand)}
.btn.primary{background:var(--brand);border-color:var(--brand);color:var(--brand-ink)}
.btn.primary:hover{filter:brightness(1.08)}
.btn.ghost{background:transparent}
.panel{background:var(--surface);border:1px solid var(--line);border-radius:var(--radius);padding:16px 18px;margin:16px 0}
.panel h2{margin:0 0 6px;font-size:1.1rem}
.hint{color:var(--muted);font-size:.92rem;margin:.2em 0 .6em}
.files{list-style:none;margin:0;padding:0;display:grid;gap:6px}
.files li{display:flex;justify-content:space-between;gap:12px;padding:8px 10px;border:1px solid var(--line);border-radius:8px}
.files li span{color:var(--muted);font-size:.85rem;white-space:nowrap}
.files a{word-break:break-all}

.notebook{margin-top:22px;border:1px solid var(--line);border-radius:var(--radius);overflow:hidden;background:#fff;box-shadow:var(--shadow)}
.nb-head{display:flex;justify-content:space-between;gap:10px;padding:10px 16px;background:#f3f5f9;border-bottom:1px solid #e3e6ef;font-size:.9rem;color:#5b6478}
.nb-head a{color:#1f4fd8}
.nb-body{padding:8px 20px 20px;color:#151a2d;overflow-x:auto}
.nb-body .cell{margin:12px 0}
.nb-body .rendered_html h1{font-size:1.5rem;margin:1.1em 0 .4em;border-bottom:1px solid #e3e6ef;padding-bottom:.25em}
.nb-body .rendered_html h2{font-size:1.25rem;margin:1em 0 .3em}
.nb-body .rendered_html h3{font-size:1.08rem}
.nb-body .anchor-link{opacity:0;margin-left:.3em;font-size:.8em;color:#9aa3b8}
.nb-body h1:hover .anchor-link,.nb-body h2:hover .anchor-link,.nb-body h3:hover .anchor-link{opacity:1}
.nb-body .input_area{background:#f6f8fb;border:1px solid #e3e6ef;border-left:3px solid #1f4fd8;border-radius:8px;overflow-x:auto}
.nb-body .input_area pre{margin:0;padding:12px 14px;font:13.5px/1.5 ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;background:transparent}
.nb-body .highlight{background:transparent}
.nb-body .output_area{margin-top:6px;overflow-x:auto}
.nb-body .output_area pre{margin:0;padding:8px 14px;font:13px/1.45 ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;white-space:pre-wrap;word-break:break-word;color:#333}
.nb-body .output_stderr pre{background:#fff4f4;color:#a12a2a;border-radius:6px}
.nb-body .output_area img{display:block;margin:6px 0;height:auto}
.nb-body table.dataframe{border-collapse:collapse;font-size:12.5px;margin:6px 0}
.nb-body table.dataframe th,.nb-body table.dataframe td{border:1px solid #e3e6ef;padding:4px 8px;text-align:right;white-space:nowrap}
.nb-body table.dataframe thead th{background:#f3f5f9}
.nb-body table.dataframe tbody tr:nth-child(odd){background:#fafbfd}
.nb-body .rendered_html table{border-collapse:collapse}
.nb-body blockquote{border-left:3px solid #e3e6ef;margin:0;padding-left:12px;color:#5b6478}

.pager{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin-top:26px}
.pager a{background:var(--surface);border:1px solid var(--line);border-radius:var(--radius);padding:12px 14px;color:var(--ink);font-weight:600;font-size:.95rem}
.pager a small{display:block;color:var(--muted);font-weight:500}
.pager a.next{text-align:right}
.pager a:hover{border-color:var(--brand);text-decoration:none}

.prose{max-width:760px;padding-top:24px;padding-bottom:40px}
.footer{border-top:1px solid var(--line);margin-top:40px;padding:22px 0;color:var(--muted);font-size:.9rem}

@media (max-width:640px){
  .hero{padding:32px 0 28px}
  .topbar nav a:not(.yt){display:none}
  .grid{grid-template-columns:1fr}
  .pager{grid-template-columns:1fr}
  .pager a.next{text-align:left}
  .nb-body{padding:6px 10px 14px}
  .btn{flex:1 1 auto;justify-content:center}
}

/* ---- v2: SEO / engagement additions ---- */
.skip{position:absolute;left:-999px;top:8px;background:#fff;padding:8px 12px;border-radius:8px;z-index:99}
.skip:focus{left:8px}
.lede{font-size:1.08rem;color:var(--muted);max-width:820px;margin:-4px 0 16px}
.lite-yt{cursor:pointer;display:block}
.lite-yt img{position:absolute;inset:0;width:100%;height:100%;object-fit:cover}
.lite-yt .play-btn{position:absolute;left:50%;top:50%;transform:translate(-50%,-50%);background:none;border:0;cursor:pointer;opacity:.9;transition:opacity .15s,transform .15s}
.lite-yt:hover .play-btn{opacity:1;transform:translate(-50%,-50%) scale(1.08)}
button.btn{cursor:pointer;font:inherit;font-weight:600}
.done-toggle.on{background:#e8f6ec;border-color:#2e9d57;color:#1d6b3a}
.facts{list-style:none;padding:0;margin:4px 0 12px;display:flex;flex-wrap:wrap;gap:8px 18px;font-size:.92rem;color:var(--muted)}
.facts b{color:var(--ink);margin-right:6px;font-weight:600}
.tags{display:flex;flex-wrap:wrap;gap:6px;margin:0 0 12px}
.tag{font-size:.8rem;padding:3px 10px;border-radius:999px;background:var(--surface);border:1px solid var(--line);color:var(--ink)}
a.tag:hover{border-color:var(--brand);text-decoration:none}
.two-col{display:grid;grid-template-columns:1fr 300px;gap:16px;align-items:start}
.two-col .panel{margin:0 0 16px}
.learn ul{margin:6px 0 0;padding-left:20px}
.learn li{margin:3px 0}
.cta{background:linear-gradient(160deg,#fff7f6,#fff);border-color:#f3d2cf}
.cta p{color:var(--muted);font-size:.92rem;margin:.2em 0 .8em}
.cta .btn{width:100%;justify-content:center;margin-top:8px}
.yt-btn{background:#e62117;border-color:#e62117;color:#fff}
.yt-btn:hover{background:#c41a12;color:#fff}
@media (prefers-color-scheme:dark){.cta{background:var(--surface);border-color:var(--line)}.done-toggle.on{background:#16321f;color:#9be3b1}}
.nb-head h2{font-size:.95rem;margin:0;color:#5b6478}
.nb-body .input_area{position:relative}
.nb-body .copy{position:absolute;top:6px;right:6px;font:600 12px system-ui,sans-serif;padding:3px 9px;border-radius:6px;border:1px solid #d5dae6;background:#fff;color:#5b6478;cursor:pointer;opacity:0;transition:opacity .15s}
.nb-body .input_area:hover .copy,.nb-body .copy:focus{opacity:1}
.related{margin-top:30px}
.related h2{margin-bottom:12px}
.thumb .dur{position:absolute;left:8px;bottom:8px;background:rgba(0,0,0,.75);color:#fff;font-size:.72rem;padding:2px 6px;border-radius:5px}
.done-badge{display:none;position:absolute;top:8px;left:8px;background:#2e9d57;color:#fff;border-radius:999px;width:24px;height:24px;font-size:.85rem;line-height:24px;text-align:center;font-weight:700}
.card.is-done .done-badge{display:block}
.card.is-done .thumb img{opacity:.75}
.progress{color:#2e9d57;font-weight:600;font-size:.92rem;align-self:center}
.hero-chips{display:flex;flex-wrap:wrap;gap:8px;margin-top:16px}
.hero-chips .chip{background:rgba(255,255,255,.14);border-color:rgba(255,255,255,.25);color:#fff}
.hero-chips .chip span{color:#cdd6ff}
.why{display:grid;grid-template-columns:repeat(4,1fr);gap:14px;margin:26px 0 6px}
.why div{background:var(--surface);border:1px solid var(--line);border-radius:var(--radius);padding:14px 16px}
.why p{margin:.3em 0 0;color:var(--muted);font-size:.9rem}
.chip.big{font-size:1rem;padding:8px 16px}
.footer-inner{display:grid;gap:8px}
.footer-inner p{margin:.2em 0}
.footer .small{font-size:.82rem}
@media (max-width:860px){.two-col{grid-template-columns:1fr}.why{grid-template-columns:1fr 1fr}}
@media (max-width:640px){.why{grid-template-columns:1fr}.facts{gap:6px 12px}}

/* ---- support ---- */
.coffee-btn{background:#ffdd00;border-color:#f2cf00;color:#111!important}
.coffee-btn:hover{background:#ffe433;border-color:#e6c400;text-decoration:none}
.coffee-btn.big{font-size:1.1rem;padding:14px 22px}
.topbar nav a.support-link{color:#7a5a00;background:#fff6c2}
.topbar nav a.support-link:hover{background:#ffec80;color:#4d3900}
.support-band{display:flex;justify-content:space-between;align-items:center;gap:16px;flex-wrap:wrap;margin:26px 0;padding:20px 22px;border-radius:var(--radius);background:linear-gradient(135deg,#fff9d6,#fffdf2);border:1px solid #f3e39a}
.support-band h2{margin:0 0 4px;font-size:1.15rem;color:#151a2d}
.support-band p{margin:0;color:#5b5030;max-width:620px}
.support-actions{display:flex;gap:10px;flex-wrap:wrap}
.support-band .btn.ghost{background:#fff;color:#151a2d}
@media (prefers-color-scheme:dark){.support-band{background:#2a2614;border-color:#4d4420}.support-band h2{color:#fff3b0}.support-band p{color:#d9cfa3}.topbar nav a.support-link{background:#3a3210;color:#ffe680}}
.toast{position:fixed;right:16px;bottom:16px;z-index:50;max-width:340px;background:var(--surface);color:var(--ink);border:1px solid var(--line);border-radius:14px;box-shadow:0 12px 32px rgba(16,24,40,.22);padding:14px 40px 14px 16px;animation:toastin .25s ease-out}
.toast p{margin:0 0 10px;font-size:.92rem}
.toast-close{position:absolute;top:6px;right:8px;border:0;background:none;font-size:1.4rem;line-height:1;cursor:pointer;color:var(--muted)}
@keyframes toastin{from{transform:translateY(12px);opacity:0}to{transform:none;opacity:1}}
@media (max-width:640px){.toast{left:16px;right:16px;max-width:none}.topbar nav a.support-link{display:inline-block}}