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
An Interface is used to specify what an object does without specifying how it does it.
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.
its more or less a method definition to make classes consistent.
add this interface to your window and you could iterate through all windows in the AppearanceChanged event
and call a method to update the Appearance.
xojo came with a example design patterns / observer