[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