If you open the Package Manager window while using the Light theme, a warning will appear in the console:

Texture not found for path: Icons/PackageManager/Light/Development.png

1. Open Unity
2. Create a new project with any settings
3. Switch to the Personal theme in Editor Preferences
3. Go to Window > Package Manager

Expected Result:
Window opens with no errors in the console.

Actual Result:
An error is produced in the console.

Tested on Windows.

Reproduced in 2019.3.0a3

Cannot Reproduce in 2019.3.0a2, 2019.2.0b3