When new Comment Reply is created, background color of new reply is incorrect.

[kirasong]

Since we're now using pretty AJAX animation when replying or approve/replying, and it is using static background colors for "default", when the CSS values for the default background color for the table/items changed, the new reply's backgroundColor became incorrect.

To reproduce:

• Go to comments screen
• Click "Reply" under a comment
• Click "Reply" or "Approve and Reply"
• Notice incorrect background color on new comment

[kirasong]

Now uses the parent widefat table background-color for the reply creation animation, instead of a static color.

[kirasong]

Refreshed patch per azaozz's most recent changes/fixes.

[azaozz]

In [18065]:

Fix background color for approved comments, props DH-Shredder, fixes #17405