"Dark Frame" theme: do not force the use of fusion style

Unlike the "dark" theme, thia theme does not try to modify
the standard widgets and dialogs.

Change-Id: If85f1bc90113516750c546cbec74cd1f562683eb
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>

share/qtcreator/themes/darkframe.creatortheme

 [General]
 ThemeName=Dark Frame
-PreferredStyles=Fusion
+PreferredStyles=
 
 [Palette]
 shadowBackground=ff404244