// Getting a CSS Variable's Value
getComputedStyle(document.documentElement) // or specific element
    .getPropertyValue('--my-variable-name'); // #999999
    
  
// Setting a CSS Variable's Value
document.documentElement.style // or specific element
    .setProperty('--my-variable-name', 'pink');
    
    
// OR
var root = document.querySelector(':root'); // = document.documentElement