UI Builder: "StyleSheets" section in light mode view has one line in different background

Steps to reproduce:

1. Create a project
2. Set "Editor Theme" to "Light" (Unity->Settings->General)
3. Window->UI Toolkit->UI Builder
4. Observe the window

Actual result: The StyleSheet section has one line in different color compared with all the background. Added screenshot in the "Attachments" section.

Expected result: Background should be the same.

Reproducible with: 2023.2.0a12 (FAV), 2023.2.0b9, 2023.3.0a5

Not reproducible with: 2021.3.30f1, 2022.3.8f1, 2023.1.11f1, 2023.2.0a11.