:root{
  --bg:#fbfcfe; --fg:#1d2330; --muted:#5b6577; --line:#e3e8f0;
  --brand:#1f5fd0; --brand2:#0f9d76; --amber:#b26b00; --red:#c0392b;
  --code-bg:#0f172a; --code-fg:#e6edf7; --code-line:#7d8ba1;
  --inline-bg:#eef3fb; --card:#ffffff;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0; background:var(--bg); color:var(--fg);
  font-family:"Source Han Sans SC","Noto Sans SC","Microsoft YaHei",system-ui,-apple-system,sans-serif;
  font-size:16px; line-height:1.75; -webkit-font-smoothing:antialiased;
}
.wrap{max-width:900px;margin:0 auto;padding:0 22px 90px}
a{color:var(--brand);text-decoration:none}
a:hover{text-decoration:underline}

/* 顶部 */
.topbar{position:sticky;top:0;z-index:30;background:rgba(251,252,254,.92);
  backdrop-filter:blur(8px);border-bottom:1px solid var(--line)}
.topbar .inner{max-width:900px;margin:0 auto;padding:10px 22px;display:flex;
  align-items:center;gap:14px;font-size:14px;color:var(--muted)}
.topbar a{color:var(--muted)}
.topbar .spacer{flex:1}
.badge{display:inline-block;padding:1px 8px;border-radius:999px;font-size:12px;
  background:var(--inline-bg);color:var(--brand);border:1px solid #d6e2f7}
.badge.star{background:#fff7e6;color:#a86a00;border-color:#f3d9a4}
.badge.diff{background:#eefaf4;color:#0b7f5f;border-color:#bfe8d9}

h1{font-size:30px;line-height:1.3;margin:30px 0 8px;letter-spacing:-.3px}
.sub{color:var(--muted);font-size:15px;margin-bottom:22px}
h2{font-size:22px;margin:38px 0 10px;padding-bottom:6px;border-bottom:2px solid var(--line)}
h3{font-size:17.5px;margin:24px 0 8px}
h4{font-size:15.5px;margin:18px 0 6px;color:var(--muted)}
p{margin:10px 0}

/* 元信息卡 */
.meta{display:flex;flex-wrap:wrap;gap:8px;margin:6px 0 18px}
.meta span{font-size:13px;color:var(--muted);background:#fff;border:1px solid var(--line);
  border-radius:8px;padding:3px 9px}

/* 代码 */
pre{background:var(--code-bg);color:var(--code-fg);border-radius:10px;padding:14px 16px;
  overflow-x:auto;font-size:13.5px;line-height:1.62;margin:12px 0;
  font-family:"JetBrains Mono",Consolas,"Cascadia Mono",Menlo,monospace;tab-size:4}
pre .c{color:#7f8fa6;font-style:normal}         /* 注释 */
pre .k{color:#7aa2f7}                            /* 关键字 */
pre .s{color:#9ece6a}                            /* 字符串/字符 */
pre .n{color:#ff9e64}                            /* 数字 */
pre .f{color:#bb9af7}                            /* 函数名 */
code{font-family:"JetBrains Mono",Consolas,Menlo,monospace;font-size:.92em;
  background:var(--inline-bg);border-radius:4px;padding:1px 5px;color:#1b3a6b}
pre code{background:none;padding:0;color:inherit;font-size:inherit}

/* 提示框 */
.box{border-radius:10px;padding:12px 16px;margin:14px 0;border:1px solid;background:#fff}
.box .t{font-weight:700;margin-bottom:4px;font-size:14.5px}
.box ul{margin:6px 0 0 18px}
.box p{margin:4px 0}
.box.tip{background:#eef6ff;border-color:#c9defb}
.box.tip .t{color:#1b5ab5}
.box.warn{background:#fff6ec;border-color:#f6d7ae}
.box.warn .t{color:#a76500}
.box.ok{background:#eefaf3;border-color:#c3e9d6}
.box.ok .t{color:#0b7f5f}
.box.info{background:#f5f6f9;border-color:#e0e4ec}
.box.info .t{color:#4a5568}

/* 例题卡 */
.prob{border:1px solid var(--line);border-left:4px solid var(--brand);border-radius:10px;
  background:#fff;padding:14px 18px;margin:16px 0}
.prob .ph{display:flex;flex-wrap:wrap;align-items:center;gap:10px;margin-bottom:8px}
.prob .pn{font-weight:700;font-size:16.5px}
.prob .oi{font-size:13px;color:var(--muted);background:var(--inline-bg);
  border-radius:6px;padding:2px 8px}
.prob .st{font-size:15px}
.prob .io{background:#f8fafc;border:1px dashed #d7dfeb;border-radius:8px;padding:8px 12px;
  margin:8px 0;font-family:"JetBrains Mono",Consolas,monospace;font-size:13px;white-space:pre-wrap}

/* 表格 */
table{border-collapse:collapse;width:100%;margin:12px 0;font-size:14.5px}
th,td{border:1px solid var(--line);padding:7px 10px;text-align:left;vertical-align:top}
th{background:#f2f6fd;color:#274b8a;font-weight:700}
tbody tr:nth-child(even){background:#fafbfd}

/* 列表 */
ul,ol{margin:10px 0 10px 22px;padding:0}
li{margin:5px 0}

/* 分块代码标题 */
.cap{font-size:13.5px;color:var(--muted);margin:14px 0 -6px;font-weight:600}

/* 逐行说明表 */
.lines td:first-child{font-family:"JetBrains Mono",Consolas,monospace;white-space:pre-wrap;
  color:#1b3a6b;width:44%}

/* 教学区块 */
.step{border-left:3px solid #cfdcf5;margin:20px 0;padding:2px 0 2px 16px}
.step>.sh{font-size:15px;font-weight:700;color:#274b8a;margin-bottom:4px}
.quote{border-left:4px solid var(--brand2);background:#f2fcf8;padding:10px 16px;
  border-radius:0 8px 8px 0;margin:16px 0;color:#0d6b52;font-weight:600}
.reproduce{background:#fffdf5;border:1px dashed #e8d9a8;border-radius:10px;padding:12px 16px;
  margin:16px 0}
.reproduce .rt{font-weight:700;color:#8a6d00;margin-bottom:4px}

/* 新函数卡 */
.fn{border:1px solid #d9e2f2;border-radius:10px;background:#fff;margin:14px 0;overflow:hidden}
.fn .fh{background:#f2f6fd;padding:10px 16px;font-weight:700;color:#274b8a;font-size:15px}
.fn .fb{padding:10px 16px}
.fn .sig{font-family:"JetBrains Mono",Consolas,monospace;font-size:13.5px;background:#0f172a;
  color:#e6edf7;border-radius:8px;padding:10px 12px;margin:4px 0 10px;white-space:pre-wrap}

/* 挑战题 */
.challenge{border:2px solid #f0d9a8;border-radius:12px;background:#fffdf7;padding:4px 20px 16px;
  margin:26px 0}
.challenge>.ct{font-size:19px;font-weight:800;color:#8a5a00;margin:14px 0 4px}
.hintbox{background:#fff8e8;border:1px solid #f0dcb0;border-radius:8px;padding:10px 14px;margin:12px 0}
.hintbox .ht{font-weight:700;color:#8a5a00;font-size:14px;margin-bottom:3px}
details{border:1px solid var(--line);border-radius:8px;background:#fff;margin:10px 0;padding:0 14px}
details>summary{cursor:pointer;padding:10px 0;font-weight:700;color:#274b8a;list-style:none}
details>summary::-webkit-details-marker{display:none}
details>summary::before{content:"▸ ";color:var(--brand)}
details[open]>summary::before{content:"▾ "}
details .dc{padding:0 0 12px}

/* 数学补给 */
.mathcard{background:linear-gradient(180deg,#f7f9ff,#fff);border:1px solid #dce5f6;
  border-radius:12px;padding:4px 20px 16px;margin:24px 0}
.mathcard>.mt{font-size:17px;font-weight:800;color:#274b8a;margin:14px 0 2px}

/* 思考点 */
.think{background:#f6f4ff;border:1px solid #ddd4fb;border-radius:12px;padding:4px 20px 14px;margin:24px 0}
.think>.tt{font-size:17px;font-weight:800;color:#4b3a9e;margin:14px 0 2px}

/* 页脚导航 */
.nav{display:flex;justify-content:space-between;gap:12px;margin:44px 0 0;
  border-top:1px solid var(--line);padding-top:18px;font-size:14.5px}
.nav a{background:#fff;border:1px solid var(--line);border-radius:9px;padding:9px 14px;
  flex:1;text-align:center}
.nav a.disabled{opacity:.4;pointer-events:none}

/* 目录页 */
.toc-group{margin:26px 0 8px;font-size:18px;font-weight:800;color:#274b8a;
  border-left:5px solid var(--brand);padding-left:10px}
.toc-list{display:grid;grid-template-columns:1fr;gap:6px}
@media(min-width:720px){.toc-list{grid-template-columns:1fr 1fr}}
.toc-item{display:flex;gap:10px;align-items:baseline;background:#fff;border:1px solid var(--line);
  border-radius:9px;padding:9px 12px;font-size:14.5px}
.toc-item:hover{border-color:#b9cdf0;text-decoration:none}
.toc-item .no{color:var(--muted);font-size:12.5px;min-width:34px;font-weight:700}
.toc-item .nm{flex:1;color:var(--fg)}
.toc-item.done .nm{color:#334;}
.stars{color:#e0a300;font-size:12px}
.legend{font-size:13.5px;color:var(--muted);background:#fff;border:1px solid var(--line);
  border-radius:9px;padding:10px 14px;margin:14px 0}
.progressbox{background:#fff;border:1px solid var(--line);border-radius:10px;padding:12px 16px;margin:16px 0}
.bar{height:9px;background:#e9eef7;border-radius:99px;overflow:hidden;margin-top:8px}
.bar>i{display:block;height:100%;background:linear-gradient(90deg,#1f5fd0,#0f9d76)}
.toc-search{width:100%;padding:10px 14px;border:1px solid var(--line);border-radius:10px;
  font-size:15px;margin:12px 0;background:#fff;color:var(--fg)}
.kbd{font-family:Consolas,monospace;background:#eef1f6;border:1px solid #dde3ec;
  border-bottom-width:2px;border-radius:5px;padding:0 5px;font-size:13px}
footer{margin-top:50px;color:var(--muted);font-size:13.5px;text-align:center}
@media print{.topbar,.nav{display:none}body{background:#fff}pre{background:#f6f8fa;color:#111}
  .box,.prob,.challenge{break-inside:avoid}}