Active USS StyleSheet name is not bolded after setting it as active in UI Builder

Reproduction steps:
1. Create a new project
2. Create UI Document Asset (Create > UI Toolkit > UI Document)
3. Double-click newly created Asset to open UI Builder window
4. Create two new USS StyleSheets in the UI Builder window and save changes
5. Right-click on one of the non-bolded StyleSheets and select “Set as Active USS” option
6. Observe both StyleSheets

Actual result: Active USS StyleSheet name is not bolded
Expected result: Active USS StyleSheet name is bolded

Reproducible with: 6000.0.0f1, 6000.0.76f1, 6000.3.16f1, 6000.4.9f1, 6000.5.0b10, 6000.6.0a6

Reproduced on: Ubuntu 24.04, macOS Tahoe 26.4 (M4)

Note: Active USS StyleSheet name becomes bold after expanding/collapsing USS StyleSheets