:root {
  --bg: #fdfdfc;
  --text: #24292f;
  --muted: #57606a;
  --accent: #8c1515;
  --border: #e4e4e1;
  --code-bg: #f6f6f4;
  --card-bg: #ffffff;
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  font-family: "PingFang SC", "Hiragino Sans GB", "Noto Sans CJK SC", "Source Han Sans SC", -apple-system, "Segoe UI", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.8;
  font-size: 17px;
}
.container { max-width: 860px; margin: 0 auto; padding: 24px 20px 80px; }

/* 顶部导航 */
.topnav {
  position: sticky; top: 0; z-index: 10;
  background: rgba(253, 253, 252, 0.92);
  backdrop-filter: blur(6px);
  border-bottom: 1px solid var(--border);
  display: flex; align-items: center; justify-content: space-between;
  padding: 10px 20px;
}
.topnav .brand { font-weight: 700; color: var(--accent); text-decoration: none; font-size: 16px; }
.topnav .crumb { color: var(--muted); font-size: 14px; }

h1 { font-size: 1.9em; margin: 1.2em 0 0.4em; line-height: 1.4; }
h2 {
  font-size: 1.4em; margin: 2.2em 0 0.6em;
  padding-bottom: 0.25em; border-bottom: 1px solid var(--border);
}
h3 { font-size: 1.15em; margin: 1.6em 0 0.4em; }
p { margin: 0.7em 0; }
a { color: var(--accent); }
.subtitle { color: var(--muted); margin-top: 0; }

/* 摘要框 */
.lecture-meta {
  background: var(--card-bg); border: 1px solid var(--border); border-radius: 10px;
  padding: 14px 18px; margin: 1.2em 0;
}
.lecture-meta b { color: var(--accent); }

/* 重点讲解块 */
.deep-dive {
  background: #fbf7f0; border-left: 4px solid var(--accent);
  border-radius: 0 8px 8px 0; padding: 14px 18px; margin: 1.4em 0;
}
.deep-dive .tag {
  display: inline-block; font-size: 12px; font-weight: 700; color: #fff;
  background: var(--accent); border-radius: 4px; padding: 1px 8px; margin-bottom: 6px;
  letter-spacing: 1px;
}
/* 拓展块 */
.extension {
  background: #f0f6f3; border-left: 4px solid #2e7d5b;
  border-radius: 0 8px 8px 0; padding: 14px 18px; margin: 1.4em 0;
}
.extension .tag {
  display: inline-block; font-size: 12px; font-weight: 700; color: #fff;
  background: #2e7d5b; border-radius: 4px; padding: 1px 8px; margin-bottom: 6px;
  letter-spacing: 1px;
}
/* 关键概念 */
.keypoint {
  background: #f4f1fa; border-left: 4px solid #5b4b8a;
  border-radius: 0 8px 8px 0; padding: 12px 18px; margin: 1.2em 0;
}

/* 数学公式 */
.formula {
  text-align: center; font-family: "STIX Two Math", Georgia, serif;
  font-size: 1.1em; margin: 1em 0; overflow-x: auto;
}

code {
  font-family: "SF Mono", Menlo, Consolas, monospace;
  background: var(--code-bg); border-radius: 4px; padding: 1px 6px; font-size: 0.88em;
}
pre {
  background: var(--code-bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 14px 16px; overflow-x: auto; line-height: 1.6;
}
pre code { background: none; padding: 0; font-size: 0.9em; }

blockquote {
  margin: 1em 0; padding: 4px 18px; color: var(--muted);
  border-left: 3px solid var(--border);
}

table { border-collapse: collapse; width: 100%; margin: 1.2em 0; font-size: 0.95em; }
th, td { border: 1px solid var(--border); padding: 8px 12px; text-align: left; }
th { background: var(--code-bg); }

figure { margin: 1.4em 0; text-align: center; }
figcaption { color: var(--muted); font-size: 0.9em; margin-top: 6px; }

/* 上一讲/下一讲 */
.pager {
  display: flex; justify-content: space-between; gap: 12px;
  margin-top: 3em; border-top: 1px solid var(--border); padding-top: 18px;
}
.pager a {
  text-decoration: none; color: var(--text);
  background: var(--card-bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px 16px; max-width: 48%;
}
.pager a:hover { border-color: var(--accent); }
.pager .dir { display: block; font-size: 12px; color: var(--muted); }

/* 首页课程列表 */
.lecture-list { list-style: none; padding: 0; margin: 1.5em 0; }
.lecture-list li {
  background: var(--card-bg); border: 1px solid var(--border); border-radius: 10px;
  margin-bottom: 10px; transition: border-color .15s;
}
.lecture-list li:hover { border-color: var(--accent); }
.lecture-list a {
  display: flex; align-items: baseline; gap: 14px;
  text-decoration: none; color: var(--text); padding: 12px 18px;
}
.lecture-list .num { font-weight: 700; color: var(--accent); min-width: 2.2em; }
.lecture-list .desc { color: var(--muted); font-size: 0.9em; margin-left: auto; text-align: right; }