AI Settings: Spinner visibility is insufficient and needs improvement

Steps to reproduce:

1. Create a Unity project
2. Select "Light" Unity theme
3. Click the "AI" button
4. Click "Agree and install Unity AI"
5. Wait until the installation is complete
6. Open the Assistant window
7. Click the "Settings" button, which should be displayed next to the "+" button
8. Click "Refresh Project Overview"
9. Observe the result

Actual results: Spinner is poorly visible 

Expected results: The spinner must be clearly visible

Reproducible with versions: 2.4.0-pre.1

Tested on (OS): macOS - Tahoe 26.4