How to make Main Screen transparent in BlackBerry?

public final class LoadingPopupScreen extends PopupScreen
{
    Background  background;
    HorizontalFieldManager hfm;
    int width;
    int height;
    public LoadingPopupScreen( int width, int height)
    {  
        super(new VerticalFieldManager());
        background = BackgroundFactory.createLinearGradientBackground(Color.ROYALBLUE, Color.BLACK, Color.ROYALBLUE, Color.BLACK);
        setBackground(background);
        add(new LabelField("ROAMING"));
        add(new SeparatorField());
        add(new SeparatorField());
        hfm = new HorizontalFieldManager();
        Bitmap bitmap = Bitmap.getBitmapResource("splash.png");
        BitmapField  bit= new BitmapField(bitmap);
        hfm.add(bit);
        hfm.add(new LabelField("want to check In Roaming or not?"));
        add(hfm);
        hfm = new HorizontalFieldManager(Manager.FIELD_HCENTER);
        ButtonField ok = new ButtonField("OK", HorizontalFieldManager.FIELD_HCENTER);
        ButtonField cancel = new ButtonField("CANCEL", HorizontalFieldManager.FIELD_HCENTER);
        hfm.add(ok);
        hfm.add(cancel);
        add(hfm);
    }
}