Steps to reproduce:
1. Create a new Unity project using the Universal 3D template
2. Open the Project Settings window (Edit > Project Settings)
3. Open the UI Toolkit tab in the Project Settings window
4. Change the "Runtime” property to “Default.tss” from “Default.tss (Default)”
5. Observe the field

Actual results: Changing Runtime field to Default.tss (Default) or Default.tss does nothing, field doesn’t update, always stays as “Default.tss (Default)
Expected results: Runtime field should update or menu item should only have one “Default.tss” selection

Reproducible with versions: 6000.4.0a3
Can’t test with versions: 6000.0.60f1, 6000.2.8f1, 6000.3.0b7 (the feature is only in 6000.4.0a3)

Tested on (OS): Windows 11