[Redesign] Lots of minor color changes

Summary: Ref T8099. Makes subheaders in object lists darker with a background. Moves to lighter icons in the header with a dark instead of blue hover state. Gives a nicer blue selected state for sidenavs.

Test Plan: Just eyeballing a lot of this, but with some level of consistency.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13065