[Redesign] Tweak bluebackground to be more useful

Summary: Ref T8099, Makes bluebackground more useful, add to hovercards, documents

Test Plan: Hovercards, Phriction

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D13450