Notification Comment details: set background color to fix transition oddity

[ScoutHarris]

Ref: #17790, #18064 (review)

This sets a background color on NotificationCommentDetailViewController to fix the issue where the Notifications list could be seen as the Comment loading view was transitioning in.

⚠️ Auto merge is enabled. ⚠️

To test:

• Go to Notifications.
• Select a Comment notification where the comment is not cached.
• Verify, as the loading view is transitioning in, the Notifications list cannot be seen underneath.

Regression Notes

1. Potential unintended areas of impact
   N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   N/A

3. What automated tests I added (or what prevented me from doing so)
   N/A

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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