New Post: Problems with MessageBox.ButtonRegionBackground in Windows7

<Grid>
        <Button Content="TEST"
                Width="150"
                Height="25"
                Click="Button_Click_1"/>

        <xctk:WindowContainer>
            <xctk:MessageBox x:Name="_messageBox"
                            Caption="MessageBox" 
                            Text="This is a MessageBox."
                            ButtonRegionBackground="Orange" />
        </xctk:WindowContainer>       
</Grid>

 public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            _messageBox.ShowMessageBox();
        }
    }