add css var with js code example

Example 1: set css variable from javascript

document.documentElement.style.cssText = "--tab-count: 3";
/or/
document.documentElement.style.setProperty("--tab-count", 5);
/or/
document.documentElement.setAttribute("style", "--tab-count: 5");

Example 2: js set css variable

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