[Redesign] Update Object Selector UI

Summary: Ref T8099, Better colors and spacing on the Object Selector.

Test Plan: Hover and test new spacing.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13524