hot to get and set values to css variables in js code example

Example 1: how to change a css variable with javascript

var root = document.querySelector(':root');
root.style.setProperty('--variable', 'lightblue');
/or/
root.style.setProperty('--variable', myColor);

Example 2: js set css variable

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