Answers for "change javascript variable with css"

js set css variable

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

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");
});