UI Builder leaks "DefaultCommonDark_inter.uss" stylesheet into UI Documents

How to reproduce:

1. Open the attached project (IN-5049.zip)
2. Make sure “SampleScene” is open from the “Assets/Scenes” directory
3. Observe the Game view and the text present in the UI panel on the left
4. In the Project panel go to “Assets/GUI/Theme“ and open the ”base-theme” in a text editor
5. Comment out lines 5 and 6 (starting with “-unity-font“ and “-unity-font-definition“ respectively)
6. Return to the Editor and observe the Game view

Expected result: all text is rendered as it was before

Actual result: all the UI text is invisible

Reproducible with: 2021.3.6f1, 2022.1.10f1, 2022.2.0b3, 2023.1.0a4

Could not test with: 2020.3.37f1 (UI documents could not be loaded)

Reproducible on: MacOS 12.4, Ubuntu 20.04, Windows 10

Notes:

1. Related forum thread: https://forum.unity.com/threads/unity-ui-styles-leak-into-ui-builder.1285865/
2. The UI Builder interface leaks the “DefaultCommonDark_inter.uss” stylesheet into the user's UI Document making the representation in the UI Builder different from that of the Game view. The Editor font is leaked onto all elements when they won’t possess the fonts inherently and thus all text isn’t rendered in Game view.
3. The issue appears to not be present when using a theme that inherits the “default.uss” stylesheet, however when you don’t, like in the attached project the issue becomes apparent.