Ensure styling + responsiveness matches UX requirements (review task)

Summary

This is part of a bigger effort to implement a file tree browser (FTB) in the repository.

As part of this issue we should:

• Review the styling of the repository FTB (Collaborate with UX)
• Fix any styling issues identified as part of the review, open follow-ups for larger issues or scope creep
• Styling should match mockups
• Check the feature on all supported browsers

Mockups

File	Tree

Remaining issues and known bugs

Refine lg viewport panel for file browser tree (#569507)

File tree browser focus and keyboard navigation... (#568845 - closed)

Keyboard navigation and focus management for fi... (#561563)