I'm not aware of a built-in way to do that, but it makes sense that if you are installing a dark image behind your toolbars you may want a dark theme in other areas. Possibly a change for the future?

At this time, using custom style rules you can manipulate the colors in many areas of Firefox. The following page is the work of an author who seems pretty responsive to requests, so you might find a solution to how to merge the rules with your preferred background image.

https://github.com/overdodactyl/ShadowFox/blob/master/README.md

Alternately, there might be a way to create a custom rule to insert the image into the regular dark theme without changing anything else. I don't have time to experiment with that right now... a good site to get custom style rules solutions is:

https://www.reddit.com/r/FirefoxCSS/

Thanks for the reply, I figured that didn't exist yet but shall look hopefully towards the future. Thanks for the reply, I will try installing "shadow fox" it looks really interesting! Really helpful