past css to javascript variable code example

Example 1: how to change css variable in javascript

document.documentElement.style.setProperty("--main-background-color", "green");

Example 2: change css variable with javascript

let root = document.documentElement;

root.addEventListener("mousemove", e => {
  root.style.setProperty('--mouse-x', e.clientX + "px");
  root.style.setProperty('--mouse-y', e.clientY + "px");
});