R1:7cffe557ac24

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…