div.active-background > .image-container.active > .stack > span > img,
div.active-background > span.active > img {
    --flash-duration: calc(var(--frame-delay, 5s) / 2.0);
    /*animation: flasher 1s linear 1 var(--flash-duration);*/
    animation-name: flasher;
    animation-duration: 1s;
    animation-timing-function: linear;
    animation-iteration-count: 1;
    animation-delay: var(--flash-duration);
    animation-direction: normal;
}

@keyframes flasher {
    0% {
        filter: invert(100%) saturate(0%);
    }
    70% {
        filter: invert(0%) saturate(50%);
    }
    100% {
        filter: invert(0%) saturate(100%);
    }
}