this.dataForm.GroupGenerated += (sender, eventArgs) =>
{
    eventArgs.Group.HeaderLabelStyle = new Style(typeof(Label))
    {
        Setters =
        {
            new Setter
            {
                Property = Label.HorizontalTextAlignmentProperty,
                Value = TextAlignment.Center
            },
            new Setter
            {
                Property = Label.VerticalTextAlignmentProperty,
                Value = TextAlignment.Center
            }
        }
    };
};