@keyframes shadowPulse {
    33% {
        background: #FFF;
        box-shadow: -24px 0 black, 24px 0 #FFF;
    }

    66% {
        background: black;
        box-shadow: -24px 0 #FFF, 24px 0 #FFF;
    }

    100% {
        background: #FFF;
        box-shadow: -24px 0 #FFF, 24px 0 black;
    }
}