How to remove transparent background for a child Widget ?

[Kart]

I am working in Qt5.9.1 with with linuxfb platform. I have a widget with multiple UI files. The first widget screen will be a basic home screen with some buttons. Each button click will open a new child widget screen.
But the newly opened widget has a transparent background. I tried removing this by

Obj->setAutoFillBackground(true);

This is not making any difference.
The windowOpacity parameter in Qt Designer is 1.000000.
stylesheet has been set to background-color: rgb(0, 0, 0);

What do i set the background TRUE ?

Thank you for your time.