Steps to reproduce:
1. Create new Unity project (use URP empty project)
2. Make sure Editor theme is set to Light
3. In Hierarchy window create Plane GameObject
4. Open Lighting window (Window > Rendering > Lighting)
5. Press “Generate Lighting” button and wait for light to generate
6. Select “Baked Lightmaps” tab
7. Below “Baked Lightmaps” press on “View” button to open Viewer window
8. Select “Baked Shadowmask” mode
9. Observe the text in the window  

Actual results: Text in “Baked Shadowmask” mode is difficult to read in Light Editor theme

Expected results: Text in “Baked Shadowmask” mode should be readable when Editor theme is set to Light

Reproducible with versions: 6000.0.57f1, 6000.2.3f1, 6000.3.0b1

Tested on (OS): macOS Silicon, Sequoia 15.6 (M1)