:root { --ink:#171717; --body:#4d4d4d; --mute:#737373; --line:#ebebeb; --canvas:#fafafa; --card:#fff; --error:#c50000; --link:#0070f3; }
        * { box-sizing:border-box; }
        body { margin:0; min-height:100vh; display:grid; place-items:center; padding:24px; background:var(--canvas); color:var(--ink); font-family:Arial, "Noto Sans TC", sans-serif; }
        .card { width:100%; max-width:480px; background:var(--card); border:1px solid var(--line); border-radius:16px; padding:32px; }
        .eyebrow { margin:0 0 12px; color:var(--mute); font:12px ui-monospace, monospace; letter-spacing:.04em; text-transform:uppercase; }
        h1 { margin:0; font-size:28px; letter-spacing:-.7px; }
        .subtitle { margin:12px 0 28px; color:var(--body); line-height:1.6; font-size:14px; }
        label { display:block; margin:0 0 8px; font-size:14px; font-weight:600; }
        input { width:100%; border:1px solid var(--line); border-radius:6px; padding:12px; color:var(--ink); background:#fff; font-size:15px; }
        input:focus { outline:2px solid #d3e5ff; border-color:var(--link); }
        button { width:100%; margin-top:16px; border:0; border-radius:6px; padding:12px; background:var(--ink); color:#fff; font-weight:600; cursor:pointer; }
        button:hover { background:#333; }
        .message { margin:0 0 20px; padding:12px; border:1px solid #ffd5d5; color:var(--error); background:#fff7f7; border-radius:6px; font-size:14px; }
        .challenge { margin-top:24px; padding:20px; border:1px solid var(--line); border-radius:12px; background:#fff; }
        .challenge h2 { margin:0 0 8px; font-size:16px; }
        .challenge p { margin:0 0 14px; color:var(--body); font-size:14px; line-height:1.55; }
        code { display:block; overflow-wrap:anywhere; padding:12px; border:1px solid var(--line); border-radius:6px; background:#fafafa; font:14px ui-monospace, monospace; letter-spacing:.06em; }
        .copy { margin-top:10px; background:#fff; color:var(--ink); border:1px solid var(--line); }
        .copy:hover { background:#f5f5f5; }
        .back { display:block; margin-top:24px; color:var(--link); text-align:center; font-size:14px; text-decoration:none; }
        footer { margin-top:28px; padding-top:16px; border-top:1px solid var(--line); color:var(--mute); text-align:center; font-size:12px; }