body {
    display: flex;
    flex-direction: column;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
    line-height: 1.6;
    max-width: 720px;
    margin: 2rem auto;
    padding: 0 1rem;
    color: #333;
  }
  
  header, footer {
    text-align: center;
  }
  
  h1, h2, h3 { line-height: 1.2; }
  
  a { color: #0066cc; text-decoration: none; }
  a:hover { text-decoration: underline; }
  
  ul { padding-left: 1.2rem; }

.main-nav a {
    margin: 0 0.8rem;
    color: #0066cc;
    text-decoration: none;
    font-weight: 500;
}

.main-nav a:hover {
    text-decoration: underline;
}

.main-nav a[ target="_blank" ]::after {
    content: " External";
    font-size: 0.8em;
    opacity: 0.7;
}

.site-container {
    flex: 1 0 auto;           /* 关键：让容器占满剩余高度 */
    display: flex;
    flex-direction: column;
    min-height: 100vh;         /* 确保至少占满一屏 */
}

.site-main {
    flex: 1 0 auto;           /* 内容区拉伸填满，推 footer 到底部 */
    padding: 2rem 1rem;
    max-width: 720px;
    margin: 0 auto;
    width: 100%;
}