The Flat theme did not enforce the application palette, anymore, if run
on a system in dark mode. But it should, otherwise the system palette is
applied to parts of the application and might alter the appearance.
This reinstates the mechanism.
Fixes: QTCREATORBUG-30709
Change-Id: I2698cb09c8f299bd8f733214f2b0d8236aaa4660
Reviewed-by: Eike Ziller <eike.ziller@qt.io>