Fix hovercard background color when using Dark Mode

Summary:
This CSS fix replaces the disabled hovercard hard coded white background color value by the page.content CSS variable.

Fix : T15056

Test Plan:

• Flush all Phorge caches
• Sign in
• Open any page including a reference to a closed manifest (https://we.phorge.it/D25395)
• Mouve the mouse over the closed manifest reference
• Check that hovercard background color is consistent with the theme.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25402