So, after seeing the Application.AppearanceChanged event, I thought it would be a good occasion to implement an interface in windows, to update each with different colors when dark mode. So I proceeded to the Good Old Language Reference.
After reading the relevant pages, I have to admit it does not give any idea about what to do 
http://documentation.xojo.com/api/language/interface.html
[quote]Interface
From Xojo Documentation
Language Keyword
An Interface is used to specify what an object does without specifying how it does it.
Notes
You will not use the Interface keyword directly in your code. To add an Interface to your project, use the Insert button or menu. To assign an Interface to a class, use the Interfaces button on the Inspector for the class.
See Also
Class, Implements keyword[/quote]
http://documentation.xojo.com/api/language/implements.html
[quote]Implements
From Xojo Documentation
Language Keyword
A Class can Implement an Interface.
Notes
You will not use the Implements keyword directly in your code. To have a class Implement an Interface, select the Interface using the Interfaces button in the Inspector.
See Also
Class, Interface keywords[/quote]
Fortunately, I found the ActionSourceInterfaceExample. Hopefully, that will shed some light on that mystery feature 
