[Boot2Qt] Dark mode settings in the app have no effect

MacOS 15.4.1

Qt Creator 16.0.2

Qt 6.10.0 (dev snapshot)

Device: RPi 4 6.9.10

Steps to reproduce:

1. Run quick controls gallery on Boot2Qt device
2. Change the style to dark mode in Universal or Basic style.
3. Restart the app.

Expected: The dark mode should be enabled as it is with FluentUIWin3 style.

Actual: The dark mode is not enabled. 

Reproducibility:  100%