[Feature] Support the CSS relative color syntax

[BaBrixx]

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
CSS Color Module Level 5 - Latest W3C specification draft