I've been working on a dark theme recently and it can now be used in 'Settings -> Theme'. It is not finished yet but I think it could already be helpful for you. You need to refresh the page again to see the new option in settings.
I'm not sure if it works in conjunction with your FF Addon. If you should try it, please post a screenshot here, so that I can see how it looks like on your machine ;) Thanks!
A work of art; sight for sore Eyes! Thank you! The screenshot was made with the add on enabled, showing it works without having to switch the add-on off.
I'll let you know if I run into any problems. Thanks again!!