body:before {
    background: url('https://images.cnblogs.com/cnblogs_com/guyeduxing/1823064/o_200807072535timg%20(3).jpg') center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    opacity: 0.8;
    position: fixed;
    top: 0%;
    left: -20%;
    width: 120%;
    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;
}
#home{
opacity: 0.8;
}