:root{color-scheme:light;--bg:#f5f7fa;--ink:#0f172a;--muted:#64748b;--line:#d7dee9;--card:#fff;--brand:#0b3b75;--ok:#166534}
*{box-sizing:border-box}body{margin:0;background:var(--bg);color:var(--ink);font-family:Arial,Helvetica,sans-serif}a{color:inherit;text-decoration:none}
.shell{max-width:1180px;margin:0 auto;padding:22px}.topbar{display:flex;gap:10px;margin-bottom:18px}.topbar a{background:#fff;border:1px solid var(--line);border-radius:8px;padding:9px 12px;color:#1e293b}
.hero,.lesson-head{padding:18px 0 24px}.hero h1,.lesson-head h1{font-size:34px;margin:0 0 8px}.hero p,.lesson-head p{margin:0;color:var(--muted)}
.subject-block{margin:0 0 24px}.subject-block h2{font-size:24px;margin:0 0 12px}.grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(250px,1fr));gap:14px}.lesson-card{display:flex;flex-direction:column;gap:8px;background:var(--card);border:1px solid var(--line);border-radius:8px;padding:16px;min-height:130px}.lesson-card strong{font-size:20px}.lesson-card span{color:var(--muted)}.lesson-card em{font-style:normal;color:var(--ok);font-size:14px;margin-top:auto}
.parts{display:grid;grid-template-columns:repeat(auto-fit,minmax(220px,1fr));gap:10px;margin:0 0 18px}.part-button{border:1px solid var(--line);background:#fff;border-radius:8px;padding:12px;text-align:left;cursor:pointer}.part-button.active{outline:3px solid #2563eb}.part-button strong{display:block;font-size:15px}.part-button span{display:block;color:var(--muted);font-size:13px;margin-top:4px}.part-label{font-size:14px;color:#1d4ed8;font-weight:700}
.player{display:grid;grid-template-columns:minmax(0,2fr) minmax(300px,1fr);gap:18px;align-items:start}.player video{width:100%;background:#020617;border-radius:8px;border:1px solid var(--line);aspect-ratio:16/9}.player aside{background:#fff;border:1px solid var(--line);border-radius:8px;padding:16px;min-height:320px}.counter{color:var(--muted);font-size:14px}.player h2{font-size:20px;line-height:1.25;margin:10px 0}.answer{font-weight:700;color:var(--ok)}.source{color:var(--muted);line-height:1.4}.controls{display:flex;gap:10px;margin-top:14px}.controls button{border:1px solid var(--line);background:#0f172a;color:#fff;border-radius:8px;padding:10px 14px;cursor:pointer}
.list{display:grid;grid-template-columns:repeat(auto-fit,minmax(230px,1fr));gap:10px;margin-top:20px}.qitem{border:1px solid var(--line);background:#fff;border-radius:8px;padding:12px;text-align:left;cursor:pointer}.qitem.active{outline:3px solid #60a5fa}.qitem strong{display:block;margin-bottom:6px}.qitem span{color:var(--muted);font-size:13px}
@media(max-width:850px){.player{grid-template-columns:1fr}.hero h1,.lesson-head h1{font-size:28px}.shell{padding:14px}}