R1:447ca08b1f04

Add hover state to white header icons

Summary: Adds a hover state and color for white headers. Fixes T5318

Test Plan: Test on managing a dashboard.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5318

Differential Revision: https://secure.phabricator.com/D9482