You need to have custom css enabled on your account and then have some code that looks something like this:
@keyframes rotate {
0% {transform:rotate(2deg);}
50% {transform:rotate(-2deg);}
100% {transform:rotate(2deg);}
}
.custom-rotate (the html class you want rotated) {
animation: rotate 10s infinite;
}
custom css: https://itch.io/docs/creators/css-guide
css animation: https://www.w3schools.com/css/css3_animations.asp
hope this helps!