Fix closed task hovercard background color
Summary: Ref T12600. This makes the background solid when in hovercard context.
Test Plan: Close a task, view hovercard, see solid background
Reviewers: epriestley, amckinley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T12600
Differential Revision: https://secure.phabricator.com/D17788