[dev-overlay] use css var for `<a>` tag focus ring (#76374)
### Why?
When clicking the dialog and tabbing, the `<a>` tag inside the Terminal
component was focusable but was missing an outline.
### How?
To have a unified outline for the focused `<a>` tag, use the css var
`--focus-ring`. This gives a unified style and also fixes this issue.
### Before
https://github.com/user-attachments/assets/7a3b7e45-c9e8-42f5-84fb-9ac4cf203d24
### After
https://github.com/user-attachments/assets/6309f153-1fe2-4087-a81e-49ff0e51e3fd
Closes NDX-856
