[Redesign] Uh... Add back gradient headers?

Summary: Ref T8099, hashtag#yolo. Adds back the original gradients plus a 'light' theme. Unclear which should be default, but we can play with it until a decision needs to be made.

Test Plan: Change colors a lot, turn on durable column.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13146