CSS - 背景
body:before {
content: " ";
position: fixed;
z-index: -1;
top: 0;
right: 0;
bottom: 0;
left: 0;
background: url(http://omuy6viz3.bkt.clouddn.com/2017/05/390210278.png) center 0 no-repeat;
background-size: cover;
opacity: .3;
}