.nk-main:before {
    content: "";
    display: block;
    position: fixed;
    top: 0; left: 0; width: 100%; height: 100%;
    z-index: -1;
    background: url('https://www.youtube.com/watch?v=s3NXwbTPJSo&list=PLMyVMT3MauIqeJSxeE_x4-s3pF1DFluS9&index=8') no-repeat center center;
    background-size: cover;
}