Hey,
Styling a dropdown requires javascript. There is an option to use select2 in the premium version but none of that matters. Your plugin is using the chosen js to style the select box. Since the plugin adds it it should style it. The theme can't style a select box from a plugin that is already styled by the plugin. That is backwards. If you want the plugin to be styled and the plugin author isn't working with you then I recommend developing the plugin with your own custom css.
Kadence Themes