In building fxplayground, I wanted to use colors from the site's theme in a canvas visualization. The theme colors were all stored in CSS custom properties (variables), but the visualization drawing code was all JavaScript. I needed a way to read the CSS color variables from within JavaScript code.
Luckily, there's a function called getComputedStyle that can help with that:
const color = getComputedStyle(document.documentElement).getPropertyValue("--color-primary");getComputedStyle takes a DOM node and returns a live CSSStyleDeclaration, which contains all the styles applied to that element. Calling getPropertyValue returns the value for a given property, which includes CSS variable declarations. So if there's a variable defined on the :root selector, you can get the value by calling getPropertyValue("--variable-name")!