Use CSS-modules variables in Javascript

CSS doesn't have the concept of variables like Sass does, so I'll assume you're referring to CSS custom properties, which a lot of people refer to as "CSS variables".

These custom properties are just that - custom style properties than are set to particular elements and that can be accessed at runtime by JS through getComputedStyle + getPropertyValue. Here's an example:

const root = document.documentElement;
const div = document.querySelector('div');
const x = document.getElementById('x');

const get = (elt, key) => getComputedStyle(elt).getPropertyValue(key);

console.log(
  get(root, '--foo'), // 'Foo!'
  get(div, '--bar'), // 'Bar!'
  get(x, '--baz'), // 'Baz!'
  get(x, '--foo'), // 'Foo!' because properties cascade down
)

:root {
  --foo: 'Foo!';
}
div {
  --bar: 'Bar!';
}
#x {
  --baz: 'Baz!';
}

<div></div>
<div id="x"></div>

As far as I know, there is no official way to export values from CSS files in a way similar to what you'd do in ESM (eg export const mainColor = '#ff0000';).

Sass however supports that, the syntax is as follow and requires a proper support from the sass processor you're using:

// colors.scss
:export {
  main-color: #ff0000;
}

import colors from './colors.scss';
console.log(colors['main-color']); // '#ff0000'