The appearance associated with the app's windows.
- macOS 10.14+
When the value of this property is
nil (the default), AppKit applies the current system appearance to the app's user interface elements, including its windows, views, panels, and popovers. Assigning an
NSAppearance object to this property causes the app's interface elements to adopt the specified appearance instead.
Individual windows and views may still override the app's appearance to customize their own appearance.