Check out example codes for "set css variable from javascript". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

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

Code Example 3

let docStyle = getComputedStyle(document.documentElement);

//get variable
let myVarVal docStyle.getPropertyValue('--my-variable-name');

//set variable
docStyle.setProperty('--my-variable-name', '#fff');

Code Example 4

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

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