I have a tiny proposal if you don't mind.
It would be much easier if you could make the plugin more flexible by allowing custom css following WordPress ideology. However, you could allow the plugin first look if there is a custom css called i.e. nrelate.css in it's own or the main directory. If there is no such file then apply other default css. Naturally, this would apply when a user selects the custom css option in the admin.
I appreciate your concern.