#main:before {
  content: "";
  display: block;
  background-position: center bottom;
  position: relative;
  height: 20vw;
  background-image: url("https://ce.sysu.edu.cn/bisclab/sites/default/files/%E5%AE%9E%E9%AA%8C%E5%AE%A4%E6%A6%82%E5%86%B5.jpg");
  background-size: cover;
}