.main-content {
  flex: 1;
  /* 使用占位图模拟背景，实际使用时请替换为真实图片路径 */
  background-image: url('../images/bj.jpg');
  background-size: cover;
  background-position: center;
  position: relative;
  margin-top: 60px; /* 避开顶部导航 */
  display: flex;
  justify-content: center;
  align-items: center;
}