body{
    background-image: url("https://images.cnblogs.com/cnblogs_com/aboutJava/1876974/t_201106064003u=120146849,1201696261&fm=26&gp=0.jpg");
    background-position: initial;
    background-size: cover;
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-origin: initial;
    background-clip: initial;
    height:100%;
    width:100%;
    }
#matrix{position: fixed; top:0; left: 0; z-index: -1; opacity:0.8;}