Change map theme to dark

[mserkanguzel]

Hi, first of all thank you for developing this package. We use offline map, so;

 static const String MAP_THEME_PATH_URL = "assets/render_themes/defaultrender.xml";
 static const String MAP_DARK_THEME_PATH_URL = "assets/render_themes/darkrender.xml";
 

I was using changeKey method in MapviewWidget and tried refresh but didn't works.

How can I change theme with dynamically? For example click change button or etc.