Resolve "Add border around the repository file tree"

What does this MR do?

This MR adds a border (and border radius) to the tree view when browsing files i.e. in the repository file tree.

Before MR	After MR

What are the relevant issue numbers?

Closes #50264 (closed)

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

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8534/