Resolve "Add border around the repository file tree" EE

What does this MR do?

This is a port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23018 into EE.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50264

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides