Joplin already has multiple dark themes that can be applied with the following instructions: Tools > Options > Appearance > Theme > Select a dark theme > Apply.
If the above doesn't fix your problem, create a Support thread for this issue with all appropriate form fields filled out. Upload an interface screenshot so we can see the issue you're facing. If you're using custom CSS, include it in your issue description.
Edit: The following support request seems like what you are experiencing: All my html notes have white background after v2.14.17