pass a variable to css from 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: how to change css variable in javascript

document.documentElement.style.cssText = "--main-background-color: red";