Remove new Color("--foo" [, root]) syntax

[LeaVerou]

It's trivial for people to read the computed style and create a color object with a custom property's value. We should not be handling this for them as it's quite a divergence from our focus and it complicates the constructor.

For anyone looking for what to replace calls to new Color("--foo", root) with, it's just:

new Color(getComputedStyle(root).getPropertyValue("--foo"))