:root{
  --bg:#ffffff; --bg2:#f6f8fa; --text:#1f2328; --muted:#656d76;
  --accent:#2563eb; --accent2:#1d4ed8; --border:#e5e7eb;
  --code-bg:#f6f8fa; --ok:#16a34a; --bad:#dc2626;
  --radius:10px;
}
*{box-sizing:border-box;margin:0;padding:0}
body{font-family:-apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif;background:var(--bg);color:var(--text);line-height:1.6}
[hidden]{display:none!important}
a{color:var(--accent);text-decoration:none}
a:hover{text-decoration:underline}
button{font-family:inherit;cursor:pointer}

/* topbar */
.topbar{position:sticky;top:0;z-index:40;display:flex;align-items:center;justify-content:space-between;
  padding:0 24px;height:58px;background:#fff;border-bottom:1px solid var(--border)}
.brand{font-size:20px;font-weight:800;color:var(--text);letter-spacing:-.3px}
.brand span{color:var(--accent)}
.topnav{display:flex;align-items:center;gap:22px}
.topnav a{color:var(--text);font-weight:600;font-size:15px}
.btn-chat{background:var(--accent);color:#fff;border:none;border-radius:8px;padding:9px 16px;font-size:14px;font-weight:700}
.btn-chat:hover{background:var(--accent2)}

/* hero + course grid */
.home{max-width:1080px;margin:0 auto;padding:48px 24px}
.hero{text-align:center;margin-bottom:44px}
.hero h1{font-size:36px;letter-spacing:-.5px;margin-bottom:10px}
.hero p{color:var(--muted);font-size:17px;max-width:560px;margin:0 auto}
.pill{display:inline-block;background:#eff6ff;color:var(--accent);font-weight:700;font-size:13px;
  border-radius:999px;padding:5px 14px;margin-bottom:16px}
.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(240px,1fr));gap:16px}
.card{border:1px solid var(--border);border-radius:var(--radius);padding:20px;background:#fff;cursor:pointer;
  transition:box-shadow .15s,transform .15s}
.card:hover{box-shadow:0 4px 16px rgba(0,0,0,.08);transform:translateY(-2px)}
.card .ico{font-size:26px}
.card h3{font-size:17px;margin:8px 0 4px}
.card p{color:var(--muted);font-size:13.5px;margin-bottom:12px}
.bar{height:6px;background:var(--bg2);border-radius:3px;overflow:hidden}
.bar i{display:block;height:100%;background:var(--accent);border-radius:3px;transition:width .3s}
.card .meta{font-size:12px;color:var(--muted);margin-top:6px}

/* course view */
.course{max-width:1080px;margin:0 auto;padding:32px 24px}
.crumb{font-size:13px;color:var(--muted);margin-bottom:14px}
.split{display:grid;grid-template-columns:250px 1fr;gap:28px;align-items:start}
.lessons{position:sticky;top:80px;border:1px solid var(--border);border-radius:var(--radius);overflow:hidden}
.lessons .lt{padding:12px 16px;font-weight:700;background:var(--bg2);border-bottom:1px solid var(--border)}
.lessons button{display:flex;width:100%;text-align:left;background:#fff;border:none;border-bottom:1px solid var(--border);
  padding:11px 16px;font-size:14px;color:var(--text);gap:8px;align-items:center}
.lessons button:last-child{border-bottom:none}
.lessons button:hover{background:#f0f6ff}
.lessons button.on{background:#eff6ff;color:var(--accent);font-weight:700}
.done-tick{margin-left:auto;color:var(--ok);font-weight:700}

/* lesson */
.lesson h1{font-size:28px;letter-spacing:-.4px;margin-bottom:6px}
.lesson .sub{color:var(--muted);font-size:15px;margin-bottom:26px}
.sec{margin-bottom:26px}
.sec h2{font-size:19px;margin-bottom:8px}
.sec p{font-size:15.5px;color:#333}
.note{background:#fffbeb;border:1px solid #fde68a;border-radius:var(--radius);padding:12px 16px;font-size:14.5px;margin:14px 0}
.code{position:relative;background:var(--code-bg);border:1px solid var(--border);border-radius:var(--radius);
  padding:14px 16px;margin:12px 0;overflow-x:auto}
.code pre{font-family:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;font-size:13.5px;line-height:1.55}
.code .lang-tag{position:absolute;top:8px;right:12px;font-size:11px;color:var(--muted);text-transform:uppercase;letter-spacing:.5px}
.code .copy{background:#fff;border:1px solid var(--border);border-radius:6px;font-size:11px;padding:3px 8px;color:var(--muted)}
.tok-k{color:#cf222e}.tok-s{color:#0a3069}.tok-c{color:#6e7781}.tok-n{color:#0550ae}.tok-f{color:#8250df}

/* quiz */
.quiz{border:1px solid var(--border);border-radius:var(--radius);padding:20px;margin-top:30px;background:var(--bg2)}
.quiz h2{font-size:17px;margin-bottom:14px}
.q{margin-bottom:16px}
.q p{font-weight:600;font-size:14.5px;margin-bottom:8px}
.q button{display:block;width:100%;text-align:left;background:#fff;border:1px solid var(--border);border-radius:8px;
  padding:10px 14px;font-size:14px;margin-bottom:6px;color:var(--text)}
.q button:hover{border-color:var(--accent)}
.q button.right{background:#f0fdf4;border-color:var(--ok);color:var(--ok);font-weight:700}
.q button.wrong{background:#fef2f2;border-color:var(--bad);color:var(--bad)}
.lesson-actions{display:flex;gap:12px;margin-top:26px}
.btn{background:var(--accent);color:#fff;border:none;border-radius:8px;padding:11px 20px;font-size:14.5px;font-weight:700}
.btn:hover{background:var(--accent2)}
.btn.ghost{background:#fff;color:var(--accent);border:1px solid var(--border)}
.btn:disabled{opacity:.5;cursor:default}

/* chat drawer */
.chat{position:fixed;top:58px;right:0;bottom:0;width:380px;max-width:100vw;background:#fff;
  border-left:1px solid var(--border);display:flex;flex-direction:column;z-index:50;box-shadow:-8px 0 24px rgba(0,0,0,.06)}
.chat-head{display:flex;align-items:center;gap:10px;padding:12px 16px;border-bottom:1px solid var(--border)}
.chat-head strong{font-size:15px}
.mode-row{display:flex;gap:6px;margin-left:auto}
.mode{border:1px solid var(--border);background:#fff;border-radius:8px;padding:5px 12px;font-size:12.5px;font-weight:600;color:var(--muted)}
.mode.on{background:var(--accent);border-color:var(--accent);color:#fff}
#chatClose{background:none;border:none;font-size:22px;color:var(--muted);line-height:1}
.msgs{flex:1;overflow-y:auto;padding:16px;display:flex;flex-direction:column;gap:12px}
.msg{max-width:88%;padding:10px 14px;border-radius:12px;font-size:14px;white-space:pre-wrap;word-wrap:break-word}
.msg.bot{background:var(--bg2);border:1px solid var(--border);border-bottom-left-radius:4px;align-self:flex-start}
.msg.me{background:var(--accent);color:#fff;border-bottom-right-radius:4px;align-self:flex-end}
.msg.bot pre{background:#0d1117;color:#e6edf3;border-radius:8px;padding:10px;overflow-x:auto;
  font-family:ui-monospace,Menlo,Consolas,monospace;font-size:12.5px;margin:8px 0}
.msg.bot code{font-family:ui-monospace,Menlo,Consolas,monospace;background:rgba(37,99,235,.08);border-radius:4px;padding:1px 4px;font-size:13px}
.chat-form{display:flex;gap:8px;padding:12px;border-top:1px solid var(--border)}
.chat-form input{flex:1;border:1px solid var(--border);border-radius:8px;padding:10px 12px;font-size:14px;outline:none}
.chat-form input:focus{border-color:var(--accent)}
.chat-form button{background:var(--accent);color:#fff;border:none;border-radius:8px;padding:10px 16px;font-weight:700}

@media(max-width:840px){
  .split{grid-template-columns:1fr}
  .lessons{position:static}
  .chat{width:100vw}
  .hero h1{font-size:28px}
}