If you try to use a custom QuickControls style together with a Fallback Style for controls you didn't override, the Fallback Style is not set if the application is running on iOS.
The following lines are shown within the log output when running on an iOS device.
I created a minimal sample application to illustrate the bug. If this application is run on another plattform (I tried Windows, Ubuntu Linux, macOS and android), the FallbackStyle is set correctly, and the Application has a dark background, as expected.
If you run this application on an iOS device, it shows a white Background, and a style that resembles the "Default" style more.
Note: If you use "Universal" within the setStyle()-Function, the style is correct also on the iOS device.