R1:0ec3291ff4a9

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.

| Before | After |
|---------|---------|
|{F332921}|{F332922}|

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…