[design] Introduce David's changes to colors.js

Summary: Went through what David detailed here: https://linear.app/comm/issue/DES-22/new-ui-themes and made all the changes.

Test Plan:
Went through to make sure removed colors weren't being used anywhere. Made sure changes matched @david's description.

Will kick off a release to see if anything pops up.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ginsu

Differential Revision: https://phab.comm.dev/D6469