I've found this article: http://tribulant.com/docs/wordpress-mailing-list-plugin/7890 for implement a custom theme for the newsletter plugin. But this is only for the front end.
I need the possibility to add custom editor styles to the backend and wondering why the plugin add an empty editor-style.css to the TinyMCE which cannot be overwritten with the above solution. What is the sense of an empty css file which is included, without the possibility to edit this?
I've searched this forum but cannot find a solution to embed my own styles to the TinyMCE, maybe I had overlook something and there is an action hook or something similar?