Fine-tune hunk controls block (#18543)

This PR changes the undo icon and adds a background color so that indent
lines don't bleed through the control block.

<img width="900" alt="Screenshot 2024-09-30 at 5 38 44 PM"
src="https://github.com/user-attachments/assets/4955f0f6-50ce-432f-85b9-1da0172d5e51">

Release Notes:

- N/A