Editor theme change does not apply

Steps to reproduce:

1. Open a project in 6000.7.0b1
2. Go to the Package Manager window
3. Select "Unity Registry" and search for the "Assistant" package
4. Select the "Version History" tab 
5. Find the 2.9.0-pre.2 version and click the "Update" button next to it
6. Wait for the script compilation
7. Unity -> Settings -> General and change "Editor Theme"
8. Observe the result

Actual results: The Editor UI does not change theme, and nothing tells the user why. No dialog, prompt or Console message appears

Expected results: Either the theme applies, or the user is told why it cannot and what is blocking it

Reproducible with versions: 6000.6.0a5, 6000.6.2f1, 6000.7.0b1

Not reproducible with versions: 6000.5.11f1, 6000.6.0a4

Tested on (OS): MacBook Pro - Tahoe 26.6.2