animate-bg.css

html {
    --bodycolor: burlywood;
}

.animate-bg {
    background-color: grey;
    padding: 0px;
    margin: 0px;
    animation-name: bgcolor;
    animation-duration: var(--time);
    animation-timing-function: ease;
    animation-delay: 1.5s;
    animation-iteration-count: infinite;
    animation-direction: normal;
    animation-fill-mode: forwards;
}

@keyframes bgcolor {
    0% {
        background-color: var(--start);
        opacity: 0.6;
    }
    50% {
        background-color: var(--end);
        opacity: 1;
    }
    100% {
        background-color: var(--start);
        opacity: 0.6;
    }
}