body:before {
  background: url(https://img2018.cnblogs.com/blog/1646268/201907/1646268-20190709145111387-899901278.jpg) center/cover no-repeat;
  content: '';
  background-repeat: no-repeat;
  background-position: center;
  opacity: 0.05;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  -webkit-filter: grayscale(40%);
  -moz-filter: grayscale(40%);
  -ms-filter: grayscale(40%);
  -o-filter: grayscale(40%);
  filter: grayscale(40%);
  filter: gray;
}