GH-101: Environment Variables for CSS (Update)

[wesleyboar]

Overview

• Update support for env() (new commit for Themes via Settings (Light Header Navbar) #191).
• Allow Header Redesign → FP-1458 #101 to continue to work off of important change in Themes via Settings (Light Header Navbar) #191 to relevant files.

This is the second time I've merged in GH-191 code; the first was #221.

Changes, Testing, Etc.

See #192 (as of b81f789).