.randbg {
    background: url('../_img/bg/6.jpg') no-repeat;
    -webkit-background-size: 100% 100%;
    -moz-background-size: 100% 100%;
    -o-background-size: 100% 100%;
    background-size: 100% 100%;
    /*ulterior*/
    height: 100%;
    width: 100%;
    position: absolute;
}
    .randbg.bg1 {
        background-image: url('../_img/bg/7.jpg');
    }
    .randbg.bg2 {
        background-image: url('../_img/bg/8.jpg');
    }

    .randbg.bg3 {
        background-image: url('../_img/bg/10.jpg');
    }

    .randbg.bg4 {
        background-image: url('../_img/bg/11.jpg');
    }

    .randbg.bg5 {
        background-image: url('../_img/bg/12.jpg');
    }

    .randbg.bg6 {
        background-image: url('../_img/bg/13.jpg');
    }
    .randbg.bg7 {
        background-image: url('../_img/bg/14.jpg');
    }
    .randbg.bg7 {
        background-image: url('../_img/bg/16.jpg');
    }
    .randbg.bg7 {
        background-image: url('../_img/bg/21.jpg');
    }

.randbg-overlay {
    backdrop-filter: brightness(0.5);
    position: absolute;
    height: 100%;
    width: 100%;
    z-index: 1;
}