ProgressBar {

            //...

            background: Rectangle {
                radius: 2
                color: "gray"
                implicitWidth: 200
                implicitHeight: 18
            }
}

According to this site i should be able to change the progressbar itself using this:

progress: Rectangle {
            color: "lightsteelblue"
            border.color: "steelblue"
        }

But that does not work. How do i change the style of the progressbar?