Single Detail Panel: New tab button doesn't need background-fill on resting state

Current:
<img width="788" height="56" alt="Image" src="https://github.com/user-attachments/assets/4ba4ab3a-21d5-4c3e-a9d0-cc547bc646c1" />

Expected:
<img width="792" height="58" alt="Image" src="https://github.com/user-attachments/assets/cc7fc13c-92ea-4cbd-a952-b9d18f3f6eb3" />

Notes: 
- Fill should still appear on hover, just like other icon buttons in the row