unifies mr diff file button style

What does this MR do?

This resolve the inconsistency on hovering buttons for diff file

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

Should not

Why was this MR needed?

This MR resolves #27291 (closed)

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• [] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• [ ] Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• [ ] Squashed related commits together

What are the relevant issue numbers?

Closes #27291 (closed)