toolbar: Add gap between the Kernel and REPL button (#21871)

Before:

![image](https://github.com/user-attachments/assets/dbc382a8-2ba5-4639-964f-35c934875e88)

After:

![image](https://github.com/user-attachments/assets/5faf2144-63c3-41d4-b1b8-fcd6f6fd7b7e)

Also works with dark themes:

![image](https://github.com/user-attachments/assets/1f3e9bfb-94f8-44f2-9727-e46fddccb153)

Release Notes:

- N/A

Co-authored-by: raphael.kieling <raphael.kieling-ext@ab-inbev.com>