* {
    box-sizing: border-box;
}

body {
    background: #222;
}

#box {
    width: auto;
    height: auto;
    color: #fff;
    background: #262626;
    position: relative;
    margin: 10px;
    padding: 30px;
}

#box::after,
#box::before {
    content: "";
    width: calc(100% + 4px);
    height: calc(100% + 4px);
    background: linear-gradient(45deg, #fb0094, #0000ff, #00ff00, #ffff00, #ff0000, #fb0094, #0000ff, #00ff00, #ffff00, #ff0000);
    background-size: 400%;
    position: absolute;
    top: -2px;
    left: -2px;
    z-index: -1;
    animation: animated-shadow 10s linear infinite alternate;
}

#box::after {
    filter: blur(30px);
}

@keyframes animated-shadow {
    0% {
        background-position: 0 0;
    }

    50% {
        background-position: 100% 0;
    }

    100% {
        background-position: 0 0;
    }
}