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