body {
    background: linear-gradient(-45deg, #39FF14, #000000, #8300c4);
    background-size: 400% 400%;
    animation: gradient 3s infinite;
}


@keyframes gradient {
    0% {
        background-position: 0% 50%;
    }
    50% {
        background-position: 100% 50%;
    }
    100% {
        background-position: 0% 50%;
    }
}

mark {
  background-color: blue;
}