how to change a css variable with javascript

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

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

let root = document.documentElement;

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