:root{--ink:#16161d;--muted:#4b4b57;--bg:#fbfaf7;--orange:#e2673f;
--blue:#d9e5fb;--blue-b:#8fb0e8;--green:#dff1df;--green-b:#9fcf9f;--purple:#e7defb;--purple-b:#b6a2ea;
--yellow:#fbf0c4;--yellow-b:#e2cb6b;--peach:#fbe3d3;--peach-b:#e9ab84;--cyan:#d5f1f4;--cyan-b:#83cdd6;
--pink:#fbdde3;--pink-b:#e8a0b0;--teal:#d4efe9;--teal-b:#80c6b6;--lilac:#e5e2fb;--lilac-b:#a9a2e6}
*{box-sizing:border-box}html{-webkit-text-size-adjust:100%}
body{margin:0;background:var(--bg);color:var(--ink);font-family:"Plus Jakarta Sans",system-ui,sans-serif;line-height:1.5}
a{color:inherit;text-decoration:none}
[data-lucide],svg.lucide{width:1.1em;height:1.1em;stroke-width:2;flex:none}
.tone-blue{--t:var(--blue);--tb:var(--blue-b)}.tone-green{--t:var(--green);--tb:var(--green-b)}.tone-purple{--t:var(--purple);--tb:var(--purple-b)}.tone-yellow{--t:var(--yellow);--tb:var(--yellow-b)}.tone-peach{--t:var(--peach);--tb:var(--peach-b)}.tone-cyan{--t:var(--cyan);--tb:var(--cyan-b)}.tone-pink{--t:var(--pink);--tb:var(--pink-b)}.tone-teal{--t:var(--teal);--tb:var(--teal-b)}.tone-lilac{--t:var(--lilac);--tb:var(--lilac-b)}

/* board */
.board{max-width:1280px;margin:0 auto;padding:24px 16px 40px}
.banner{text-align:center;border-radius:18px;padding:22px 16px 18px;
 background:linear-gradient(100deg,#fbe3d3 0%,#f6efe0 30%,#e4e8fb 70%,#e7defb 100%)}
.banner h1{margin:0;font-size:clamp(30px,6vw,64px);font-weight:800;letter-spacing:-.02em;line-height:1}
.banner p{margin:10px 0 0;font-size:clamp(15px,2.2vw,22px);font-weight:600;color:#2b2b35}
.hint{display:flex;gap:6px;align-items:center;justify-content:center;color:var(--muted);font-size:14px;margin:14px 0}
.grid{display:grid;grid-template-columns:1fr auto 1fr auto 1fr;gap:10px}
.col{display:grid;grid-template-rows:repeat(4,1fr);gap:14px}
.arrows{display:grid;grid-template-rows:repeat(4,1fr);gap:14px;align-items:center;color:#333}
.arrows svg{width:22px;height:22px}
.card{position:relative;display:block;background:var(--t);border:2px solid var(--tb);border-radius:10px;padding:16px 18px 34px;
 overflow:hidden;transition:transform .15s,box-shadow .15s}
.card:hover,.card:focus-visible{transform:translateY(-3px);box-shadow:0 10px 24px rgba(0,0,0,.12);outline:none}
.card h2{margin:0 0 8px;font-size:clamp(19px,1.9vw,25px);font-weight:700;letter-spacing:-.01em;position:relative;z-index:1}
.card ul{list-style:none;margin:0;padding:0;position:relative;z-index:1}
.card li{display:flex;gap:9px;align-items:center;font-size:15.5px;font-weight:500;color:#26262e;padding:2px 0}
.card li svg{opacity:.75}
.art{position:absolute;right:-14px;bottom:-14px;color:var(--tb);opacity:.45}
.art svg{width:110px;height:110px;stroke-width:1.4}
.go{position:absolute;left:18px;bottom:10px;display:flex;gap:4px;align-items:center;font-size:13px;font-weight:700;color:#333;opacity:0;transition:opacity .15s}
.card:hover .go,.card:focus-visible .go{opacity:1}
.done{position:absolute;right:12px;bottom:10px;display:none;background:#2f9e5b;color:#fff;border-radius:99px;padding:3px}
.done svg{width:14px;height:14px;stroke-width:3}
.card.seen .done{display:flex}
.formula{display:grid;gap:8px;position:relative;z-index:1;margin-top:6px}.formula .row{display:flex;gap:8px;align-items:center}
.formula .chip{display:inline-flex;gap:6px;align-items:center;background:#d4c6f6;border:1.5px solid var(--tb);border-radius:8px;padding:4px 12px;font-weight:700;font-size:16px}
.formula b{font-size:18px}
.formula .eq{font-size:22px;font-weight:800;margin-top:2px}
.cta{display:flex;gap:8px;justify-content:center;align-items:center;margin-top:16px;background:var(--orange);color:#fff;
 border-radius:8px;padding:10px 16px;font-weight:700;font-size:clamp(15px,2vw,20px)}
.cta:hover{filter:brightness(1.06)}
.fine{text-align:center;color:var(--muted);font-size:13px;margin-top:12px}
.fine button{background:none;border:0;color:inherit;text-decoration:underline;cursor:pointer;font:inherit;padding:0}
@media (max-width:900px){.grid{grid-template-columns:1fr}.arrows{display:none}.col{grid-template-rows:none}}

/* lesson */
.lesson{max-width:820px;margin:0 auto;padding:20px 16px 56px}
.crumbs{display:flex;justify-content:space-between;align-items:center;font-size:14px;color:var(--muted);font-weight:600}
.crumbs a{display:flex;gap:6px;align-items:center}.crumbs a:hover{color:var(--ink)}
.hero{display:flex;gap:18px;align-items:flex-start;margin:20px 0 14px;background:var(--t);border:2px solid var(--tb);border-radius:14px;padding:22px}
.hero-icon{background:#fff;border:2px solid var(--tb);border-radius:12px;padding:12px;display:flex}
.hero-icon svg{width:34px;height:34px}
.eyebrow{margin:0;font-size:13px;font-weight:700;text-transform:uppercase;letter-spacing:.08em;color:var(--muted)}
.hero h1{margin:2px 0 6px;font-size:clamp(28px,5vw,40px);line-height:1.1;letter-spacing:-.02em}
.tagline{margin:0;font-size:17px;color:#2b2b35}
.chips{display:flex;flex-wrap:wrap;gap:8px;margin-bottom:22px}
.chips a{display:inline-flex;gap:6px;align-items:center;background:#fff;border:1.5px solid var(--tb);border-radius:99px;padding:5px 12px;font-size:14px;font-weight:600}
.chips a:hover{background:var(--t)}
.points{display:grid;gap:12px}
.points article{background:#fff;border:1px solid #e7e4dc;border-left:5px solid var(--tb);border-radius:10px;padding:14px 18px;scroll-margin-top:16px}
.points h3{margin:0 0 4px;font-size:18px}
.points p{margin:4px 0;color:#30303a}
.try{position:relative;margin-top:22px;background:#1d1d24;color:#f2f2f5;border-radius:12px;padding:16px 18px}
.try h3{display:flex;gap:8px;align-items:center;margin:0 0 10px;font-size:16px;color:#fff}
.try pre{margin:0;white-space:pre-wrap;word-break:break-word;font:15px/1.55 ui-monospace,SFMono-Regular,Menlo,monospace;color:#e6e6ec}
.copy{position:absolute;top:12px;right:12px;display:flex;gap:6px;align-items:center;background:#33333d;color:#fff;border:0;border-radius:7px;padding:6px 10px;font:600 13px "Plus Jakarta Sans",sans-serif;cursor:pointer}
.copy:hover{background:#44444f}
.avoid{margin-top:16px;background:#fff1ee;border:1.5px solid #f2b8aa;border-radius:12px;padding:14px 18px}
.avoid h3{display:flex;gap:8px;align-items:center;margin:0 0 6px;font-size:16px;color:#b2412a}
.avoid ul{margin:0;padding-left:20px}.avoid li{margin:3px 0}
.pager{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin-top:26px}
.pager a{display:flex;gap:10px;align-items:center;background:#fff;border:1.5px solid #e2dfd6;border-radius:10px;padding:12px 14px;font-weight:700}
.pager a:hover{border-color:var(--ink)}
.pager small{display:block;font-weight:600;color:var(--muted);font-size:12px}
.pager .nx{justify-content:flex-end;text-align:right}
@media (max-width:560px){.hero{flex-direction:column}.pager{grid-template-columns:1fr}.try pre{padding-top:24px}}