jellyfin-web/dashboard-ui/bower_components/emby-webcomponents/backdrop/style.css

.backdropContainer {
    contain: layout style;
}

.backdropImage {
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
    background-attachment: fixed;
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
}