Steps to reproduce:
1. Create a UI Document (Create>UI Toolkit>UI Document)
2. Double click the document to open it
3. Dock the UI Builder window in the same tab group as the Scene view
4. Observe the the Border or the Spacing settings (also the document preview area)

Expected result: the transparent elements remain transparent
Actual result: the transparent elements turn white

Reproducible with: 2023.2.0a18, 2023.2.0b9, 2023.3.0a4
Not reproducible with: 2023.2.0a17, 2023.3.0a5

Note: In case step 3 doesn't reproduce the issue, switch to the Scene view and then switch back to the UI Builder window

Tested using Linux and Mac OS. Not tested on Windows.