Thanks for the fast reply!

I tried inserting the line

ui->Btn->setAutoFillBackground(true);

at the class initialization and before each palette change that I had to make, but with no result. I also tried with using the Role option with all Active, Inactive and Disabled.
The state of things didn't change: when the palette change is requested, only the frame changes its color.

I checked at this point if it could be a library issue and updated the existing ones. I do not get a library error message during execution, so I assumed that no graphic library is missing.

Thanks again a lot!