body:before {
    background: url(https://images.cnblogs.com/cnblogs_com/blogs/666994/galleries/1935828/o_2102230809245c198a7b3fdd3.jpg) center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    opacity: 0.79;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: -1;
    -webkit-filter: grayscale(10%);
    -moz-filter: grayscale(10%);
    -ms-filter: grayscale(10%);
    -o-filter: grayscale(10%);
    filter: grayscale(10%);
    filter: gray;
}