.body {
    position: relative;
    z-index: 1;
    background: black;
    color: white;
}
.body::before {
    content: "";
    position: absolute;
    top: 0; 
    left: 0;
    width: 100%; 
    height: 100%;
    z-index: -1;
    opacity: 0.3; 
    background: url("background.png");
}