var root = document.querySelector(':root');
root.style.setProperty('--variable', 'lightblue');
/or/
root.style.setProperty('--variable', myColor); 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");document.documentElement.style.setProperty("--tab-count", 5);element.style.setProperty("--my-var", jsVar + 4);