As I have said in others questions, the native header in Nokia SDK is not editable. This methods that you are using are invalid to modify the style of the header, the Nokia SDK don't get access to do that. And if you want to follow the Nokia guidelines in your app, you should keep it as the SDK gives to you.
Instead, if you still want to change this header, you can try to set your app in fullscreen and build a custom header on your own. Be careful, if you do that, you will lose the native navigation. If you want more info, take a look here in Stackoverflow, there are so many questions talking about that. In the Nokia developer forum, you can find many posts talking about that.