Also, this got broken after the Kirigami to verion 2.2 in main.qml.
So, set the header background color according to our theme setting.
Notice, that there is a remaining issue here. We could color the
text color in the header, but now, this seems impossible (or I
do not understand how and where to set this).
Signed-off-by: Jan Mulder <jlmulder@xs4all.nl>