h2
{
    color: greenyellow;
}
h3
{
    color: red;
}
body
{
    background:rgba(0,255,0,0.1);
}
body:before {
    background: url('https://images.cnblogs.com/cnblogs_com/hhhjjj/2136121/o_220331133843_OIP.jpg') center/cover no-repeat;
    content:"";
    opacity: 0.08;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;    
    z-index: -1;
}

canvas
{
    opacity: 0.08;
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;    
    z-index: -2;
}