Using C++.

If all the widget's backgrounds are changed by using the following qss:

QWidget:window {
    background-color: green;
} 

Then a slider track becomes visible at the right side of QColorDialog for the internal class QColorLuminancePicker. It seems to be sticking to the Qt default background:

Without the style modification for comparison:

A workaround is to add this to the qss:

QColorLuminancePicker {
    background: green;
} 

As QColorLuminancePicker is a private internal not documented class, I think it should not be needed to do this.