[redesign] Remove most gradients, tweak hovercards

Summary: The removes our least used gradients and uses base colors. Tweaked Hovercards to use.

Test Plan: Test Hovercards and UIExamples Actions Headers

Reviewers: epriestley, btrahan

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9347