Steps to reproduce:
1. Create new project
2. Window > Package Manager > + > Add package from git url: com.unity.ui.builder
3. Window > Package Manager > + > Add package from git url: com.unity.ui
4. Assets > Create > UI Toolkit > UI Document. Double NewUXMLTemplate to edit it in UI Builder
5. UI Builder > Library > Containers > Drag VisualElement onto Viewport
6. Observe that Inspector > Background Color > swatch is BLACK with 100% alpha.
7. Click on swatch, then set alpha to 90% - 100%
8. Observe that VisualElement has resized to height=0 in the Viewport
9. drag the horizontal line that represents the VisualElement in the Viewport downward to give it a height> 0
10. Observe that the VisualElement is now using the correct alpha

Expected results: Visual Element is rendered with 100 percent alpha value and has the correct size
Actual results: VIsual Element is rendered with 0 percent alpha value and sets its size to 0 height when changing alpha

Reproducible with: 2019.4.28f1(1.0.0-preview.14), 2020.3.14f1(1.0.0-preview.14), 2021.1.12f1(1.0.0-preview.14),
Unable to test with: 2021.2, 2022.1 due to package error when importing UI.builder and UI