body:before {
content: "";
display: block;
position: fixed;
left: 0;
top: 0;
width: 100%;
height: 100%;
z-index: -10;
background: url(../back.png) no-repeat center center;
filter: brightness(100%);
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}