Add white background for no readme container

What does this MR do?

Changes the readme container background to white so that it is more consistent with the other default pages

Are there points in the code the reviewer needs to double check?

Check for side effects but there really shouldn't be any

Why was this MR needed?

Improves UI

What are the relevant issue numbers?

Closes #20906 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together