Hi,
you can do it via JavaScript. Here is a solution plus a a live demo. https://www.w3resource.com/javascript-exercises/javascript-math-exercise-40.php
You will need to copy the code in the front field inside tags. For instance, the Basic template front side would be the following:
{{Front}}
<script>
function random_bg_color() {
var x = Math.floor(Math.random() * 256);
var y = Math.floor(Math.random() * 256);
var z = Math.floor(Math.random() * 256);
var bgColor = "rgb(" + x + "," + y + "," + z + ")";
console.log(bgColor);
document.body.style.background = bgColor;
}
random_bg_color();
</script>