qml中，怎么改变GroupBox的Title字体颜色？

qml中，怎么改变GroupBox的Title字体颜色？只改Title的颜色可行吗？
具体要怎么改，百度上的把其它文字内容的颜色都改了。请教CSDN各路好汉。

import QtQuick 2.4
import QtQuick.Controls 1.3
import QtQuick.Controls.Styles 1.3
import QtQuick.Controls.Private 1.0

Rectangle {
    width: 800
    height: 480
    color: "green"

    GroupBox {
        id: groupBoxId
        x: 400
        y: 300
        width: 300
        height: 100
        title: "Joining for?"

        flat : false
        style: Style {
            property Component panel: Rectangle {
                y: -15
                x: -8
                width: 300
                height: 100
                color: "transparent"
                border.color: "blue"

                Rectangle{
                    id:titleBackground
                    width: title.width
                    height: title.height
                    implicitHeight:title.height
                    color: groupBoxId.parent.color
                    x:20
                    y: -11
                    opacity:1
                    Text{
                        id:title
                        text: groupBoxId.title
                        font.pixelSize: 20
                        color:"red" //修改title 颜色
                    }
                }
            }
        }

        Column {
            spacing: 10

            CheckBox {
                text: "Breakfast"
                checked: true
            }
            CheckBox {
                text: "Lunch"
                checked: false
            }
            CheckBox {
                text: "Dinner"
                checked: true
            }
        }
    }
}