@maximo
Ok :)
I am going to test again with stylesheets.
It would be very nice it it worked as docs says.
Or at least to understand why it did not.

Update:
Ok, one must unpolish/ polish each widget as doing it to
Mainwindow doesn't seems to work.
But it also works with
setStyleSheet(styleSheet());
in MainWindow to reload (whole) style sheet.

Update 2:
Ok. Having qframes with bool error property.
It will update in Designer, if setting it.
No code. Sheet on MWindow.
So it does work with properties it seems.