The style draws different parts of widgets with different colors, e.g. to achieve a "3D" effect, see https://doc.qt.io/archives/qt-4.8/qp...ColorRole-enum

So in this case it might not be using the normal background for some of the areas.

You could try modifying the widget's palette and experiment if you can find which color role is the one being used.

Cheers,
_