how to change a variable in css code example

Example 1: css set variable

:root {
  --main-bg-color: coral;
}

#div1 {
  background-color: var(--main-bg-color);
}

#div2 {
  background-color: var(--main-bg-color);
}

Example 2: javascript use css custom properties

Values in JavaScript
To use the values of custom properties in JavaScript, it is just like standard properties.
// get variable from inline style
element.style.getPropertyValue("--my-var");

// get variable from wherever
getComputedStyle(element).getPropertyValue("--my-var");

// set variable on inline style
element.style.setProperty("--my-var", jsVar + 4);