:root{
  --bg:#f4f6fb;--ink:#151a2d;--muted:#5b6478;--card:#fff;--line:#e3e7f0;
  --c6e:#0ea5a4;--c5e:#f97316;--c4e:#7c3aed;--c3e:#e11d74;--c2nde:#2563eb;--c1ere:#16a34a;--ctle:#dc2626;
  --accent:#2563eb;--accent-2:#7c3aed;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0;background:var(--bg);color:var(--ink);font-family:"Segoe UI",system-ui,-apple-system,Roboto,Helvetica,Arial,sans-serif;font-size:17px;line-height:1.6;-webkit-font-smoothing:antialiased}
a{color:var(--accent)}
body.lv-6e{--accent:var(--c6e);--accent-2:#14b8a6}
body.lv-5e{--accent:var(--c5e);--accent-2:#fb923c}
body.lv-4e{--accent:var(--c4e);--accent-2:#a855f7}
body.lv-3e{--accent:var(--c3e);--accent-2:#f43f8e}
body.lv-2nde{--accent:var(--c2nde);--accent-2:#3b82f6}
body.lv-1ere{--accent:var(--c1ere);--accent-2:#22c55e}
body.lv-tle{--accent:var(--ctle);--accent-2:#f87171}

/* header */
header.hero{background:linear-gradient(135deg,var(--accent),var(--accent-2));color:#fff;padding:36px 18px 44px;position:relative;overflow:hidden}
header.hero::after{content:"";position:absolute;right:-80px;top:-80px;width:280px;height:280px;border-radius:50%;background:rgba(255,255,255,.14)}
header.hero::before{content:"";position:absolute;left:-60px;bottom:-120px;width:240px;height:240px;border-radius:50%;background:rgba(255,255,255,.1)}
.hero-in{max-width:1080px;margin:0 auto;position:relative;z-index:1}
.back{display:inline-flex;align-items:center;gap:6px;color:#fff;text-decoration:none;font-size:.95rem;background:rgba(255,255,255,.18);padding:6px 14px;border-radius:999px;margin-bottom:18px}
.back:hover{background:rgba(255,255,255,.3)}
h1{font-size:clamp(1.6rem,4vw,2.5rem);font-weight:800;margin:0 0 6px;letter-spacing:-.02em;line-height:1.15}
.sub{margin:0;opacity:.92;font-size:1.05rem}

main{max-width:1080px;margin:0 auto;padding:28px 18px 70px}
.intro{background:var(--card);border-left:5px solid var(--accent);border-radius:10px;padding:16px 20px;margin:0 0 28px;box-shadow:0 2px 12px rgba(21,26,45,.06)}
.intro strong{color:var(--accent)}

/* index cards */
h2.level{display:flex;align-items:center;gap:12px;font-size:1.35rem;font-weight:800;margin:36px 0 14px}
h2.level .dot{width:14px;height:14px;border-radius:50%;background:var(--lc)}
h2.level .cnt{margin-left:auto;font-size:.85rem;font-weight:600;color:var(--muted);background:#fff;border:1px solid var(--line);padding:2px 10px;border-radius:999px}
.cards{display:grid;grid-template-columns:repeat(auto-fill,minmax(230px,1fr));gap:14px}
.card{--lc:var(--accent);display:flex;flex-direction:column;gap:6px;background:var(--card);border:1px solid var(--line);border-top:5px solid var(--lc);border-radius:14px;padding:16px 18px;text-decoration:none;color:var(--ink);font-weight:600;box-shadow:0 2px 10px rgba(21,26,45,.05);transition:transform .15s,box-shadow .15s}
.card:hover,.card:focus-visible{transform:translateY(-3px);box-shadow:0 10px 24px rgba(21,26,45,.12);outline:none}
.card .num{display:inline-block;align-self:flex-start;font-size:.75rem;font-weight:700;text-transform:uppercase;letter-spacing:.05em;color:#fff;background:var(--lc);padding:2px 10px;border-radius:999px}
.card.soon{opacity:.5;pointer-events:none;border-style:dashed}

/* exercise pages */
article{background:var(--card);border:1px solid var(--line);border-radius:14px;padding:20px 22px 22px;margin-bottom:18px;box-shadow:0 2px 10px rgba(21,26,45,.05)}
article h2{display:flex;align-items:center;gap:12px;font-size:1.1rem;font-weight:700;margin:0 0 12px}
article h2 span{flex:none;display:inline-grid;place-items:center;min-width:2.2rem;height:2.2rem;padding:0 8px;border-radius:10px;background:var(--accent);color:#fff;font-size:.85rem;font-weight:800}
ol{padding-left:24px;margin:0}
li{margin-bottom:6px}
li::marker{color:var(--accent);font-weight:700}
p{margin:0 0 10px}
article p:last-child,article ol:last-child{margin-bottom:0}
table{border-collapse:separate;border-spacing:0;margin:10px 0;overflow:hidden;border-radius:8px;border:1px solid var(--line)}
td,th{border-right:1px solid var(--line);border-bottom:1px solid var(--line);padding:6px 14px;text-align:center;font-weight:normal}
tr:last-child td,tr:last-child th{border-bottom:0}
td:last-child,th:last-child{border-right:0}
th{background:color-mix(in srgb,var(--accent) 12%,#fff);font-weight:600}

/* nav between chapters */
.chapnav{display:flex;justify-content:space-between;gap:12px;margin-top:10px;flex-wrap:wrap}
.chapnav a{text-decoration:none;font-weight:600;background:#fff;border:1px solid var(--line);padding:10px 16px;border-radius:10px;color:var(--ink)}
.chapnav a:hover{border-color:var(--accent);color:var(--accent)}

/* cta */
.cta{margin-top:36px;text-align:center;background:linear-gradient(135deg,var(--accent),var(--accent-2));border-radius:16px;padding:28px 18px;color:#fff}
.cta p{margin:0 0 12px;font-weight:600;font-size:1.05rem}
.cta a{display:inline-block;padding:13px 28px;background:#fff;color:var(--accent);text-decoration:none;border-radius:999px;font-weight:800;box-shadow:0 6px 18px rgba(0,0,0,.15);transition:transform .15s}
.cta a:hover,.cta a:focus-visible{transform:translateY(-2px);outline:none}

footer{text-align:center;color:var(--muted);font-size:.9rem;padding:0 18px 30px}
footer a{color:var(--muted)}

@media (max-width:600px){
  body{font-size:16px}
  header.hero{padding:24px 16px 30px}
  main{padding:20px 14px 50px}
  article{padding:16px 14px 18px;border-radius:12px}
  .cards{grid-template-columns:1fr 1fr;gap:10px}
  .card{padding:12px 12px;font-size:.92rem;border-radius:12px}
  .intro{padding:12px 14px}
}
@media (max-width:380px){.cards{grid-template-columns:1fr}}