Mate try calling: setWindowOpacity(qreal). setting to 0 = widget is invisible, 1 = widget fully opaque 0.5 = half visible. etc...
If you are doing this on Unix/Linux apparently you have to ensure that your platform has a Compositing Window Manager. KDE4 onwards has one and recent versions of Gnome but I wouldn't know about others. There's also some other things to be mindful of on X11 platforms. If you search for "setWindowOpacity() X11 issues" you should be able to find better information.
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