Reproduction steps:
1. Create a new project
2. Create UI Document (Create > UI Toolkit > UI Document)
3. Double-click on newly created Asset to open UI Builder
4. Drag and drop Progress Bar Element to Hierarchy
5. Change the background color of the Progress Bar in the Inspector
6. Observe the Progress Bar in the viewport

Actual result: Background color does not change in the viewport
Expected result: Background color updates and displays correctly in the viewport

Reproducible with: 2022.3.69f1, 6000.0.62f1, 6000.2.13f1, 6000.3.0f1, 6000.4.0a5, 6000.5.0a1

Reproduced on: macOS Tahoe 26.1 (M4)