explicitly set colours for header items

explicitly set colours for header items

the _Universal_ theme notably sets [`icon.color` on `Button`s][0] and
also the [`color` on `Label`s][1], ignoring what's set through the
`palette` property ...

[0]: https://github.com/qt/qtdeclarative/blob/215619d71b69be02348e12dc2dfea087c3ec2abb/src/quickcontrols/universal/Button.qml#L23
[1]: https://github.com/qt/qtdeclarative/blob/215619d71b69be02348e12dc2dfea087c3ec2abb/src/quickcontrols/universal/Label.qml#L12

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>