.back {
    position: absolute;
    z-index:-1;
}

.back_bg {
    animation:slide 3s ease-in-out infinite alternate;
    background-image: linear-gradient(-60deg, #EEEEEE 50%, #FFFFFF 50%);
    bottom:0;
    left:-50%;
    opacity:.5;
    position:fixed;
    right:-50%;
    top:0;
    z-index:-1;
}
  
.back_bg2 {
    animation-direction:alternate-reverse;
    animation-duration:4s;
}
  
.back_bg3 {
    animation-duration:5s;
}

@keyframes slide {
    0% {
      transform:translateX(-25%);
    }
    100% {
      transform:translateX(25%);
    }
}