We have CSS variable as
:root {
--mouse-x: 0px;
--mouse-y: 0px;
}
.mover {
left: var(--mouse-x);
top: var(--mouse-y);
}
To update above values using Javascript, You would need to do
let root = document.documentElement;
root.addEventListener("mousemove", e => {
root.style.setProperty('--mouse-x', e.clientX + "px");
root.style.setProperty('--mouse-y', e.clientY + "px");
});