How do I set the background color of a widget like combobox or double spin box?

fhe is generally correct, but doesn’t account for the widgets (like spin boxes and buttons/comboboxes) that use a different background role in the palette. A more general solution would be something like this:

QPalette pal = widget.palette();
pal.setColor(widget.backgroundRole(), Qt::blue);
widget.setPalette(pal);

Alternatively, you could look into the descriptions of the various palette roles and figure out the one you want, then apply it to the widget containing the others you want changed. The palette changes should propagate to the children widgets.