[simple10]

Here's how to set the property value in Javascript if needed:

Get property value defined in :root{}:

getComputedStyle(document.documentElement).getPropertyValue("--some-custom-css-var");

Set property value:

document.documentElement.style.setProperty("--some-custom-css-var", "some value");

This can work as a polyfill for browsers that do not yet have CSS @property support enabled.