Background image and semitransparent widget

Hello,
I'm trying to put a background image beside some qlistview widgets. So I think that I've to made them semitransparent.
I've tried different aproaches but I can't get a semitransparent widget.
How can I do it?
Thanks in advance.

If you want a semitransparent list view the use QWidget::setWindowOpacity(0.5);

Regards

If i try the following code, I can't see the mainwindow's yellow color. But I can if I remove the setCentralWidget(a);

Code:

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QListWidget *a = new QListWidget();
setStyleSheet("QMainWindow {background-color: yellow}");
a->setWindowOpacity(0.5);
setCentralWidget(a);

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I want something like the attatched image, with designer I've selected QlistWidgets frame, as Qt::StyledFrame, and in this case it works, but i can't get it coding.