document.documentElement.setAttribute("style", "--main-background-color: green");
let docStyle = getComputedStyle(document.documentElement); //get variable let myVarVal docStyle.getPropertyValue('--my-variable-name'); //set variable docStyle.setProperty('--my-variable-name', '#fff');
document.documentElement.style.setProperty("--main-background-color", "green");
document.documentElement.style.cssText = "--tab-count: 3"; /or/ document.documentElement.style.setProperty("--tab-count", 5); /or/ document.documentElement.setAttribute("style", "--tab-count: 5");