[Feature] Support the CSS relative color syntax

### Suggestion
This would make the functionality mentioned in #40 and implemented in pull-request #89 even more powerful as one would be able to base colors on previously defined CSS variables.

An example of a use-case is:
`background: rgb(from var(--bg-color) r g b / 80%);`

_Find more information on:_
[CSS relativa color syntax - Chrome for Developers](https://developer.chrome.com/blog/css-relative-color-syntax/)
[CSS Color Module Level 5 - Latest W3C specification draft](https://www.w3.org/TR/css-color-5/#relative-colors)