body:before {
    background: url(https://images.cnblogs.com/cnblogs_com/blogs/675151/galleries/1969748/t_2104300025581856bc6cb873f81.jpg?t=1623584035990) center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    /*文本透明度*/
    opacity: 0.65;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
     z-index: -1;
    -webkit-filter: grayscale(3%);
    -moz-filter: grayscale(3%);
    -ms-filter: grayscale(3%);
    -o-filter: grayscale(3%);
    -brightness- filter:
    -webkit-filter:brightness(1.0);
    /*滤镜*/
    filter: gray;
}