Fix answer background color when using Dark Mode in Ponder

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

Test Plan:

• Flush all Phorge caches
• Sign in
• Go to question with at least one answer.
• Check that the answer block background color is consistent with the theme.
• Do these steps for each user interface theme in order to check against regression.

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/D25393