For some themes this is done anyway, but not for the default themes.
We have to do this to ensure the "base style" plays well with the applied
.css.
Otherwise the combo box might sound out in weird ways and the colors
might have usablity issues.

Task-number: QTCREATORBUG-17933
Change-Id: I4319ed9b965f4b4b3e975f2348fb89ffd9987bc3
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>