body:before {
    background: url(https://img2020.cnblogs.com/blog/2056213/202108/2056213-20210829214508607-256304377.jpg) center/cover no-repeat;
    content: '';
    background-repeat: no-repeat;
    background-position: center;
    opacity: 1.0;
    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;
}

#back-to-top {
    background-color: #fff;
    bottom: 0;
    color: #000;
    padding: 10px 10px;
    position: fixed;
    right: 50px;
    cursor: pointer;
}

#top_nav{
    display: none;
}