Feature: Relative Color Syntax

[LeaVerou]

Spec: https://drafts.csswg.org/css-color-5/#relative-colors

Articles:

• https://www.bram.us/2021/04/28/create-a-color-theme-with-css-relative-color-syntax-css-color-mix-and-css-color-contrast/
• https://blog.jim-nielsen.com/2021/css-relative-colors/
• https://dev.to/fabiogiolito/create-a-color-theme-with-these-upcoming-css-features-4o83
• https://www.stefanjudis.com/notes/new-in-css-relative-colors/

Implemented in Safari, no caniuse yet.

(Split off from #1)