Very Good. I love it however, I feel somethings could be better.
1. On the Google homepage, the images for the Google Apps, do not appear. However, if you Invert the Google homepage it appears. Not sure why this is happening but I just use invert, but some of the Google search functionality doesn't work if you use invert. [Scores of sports (NFL) wont appear if you search and click more games]

2. I would love it even more if instead of Enabling Dark backgrounds for all webpages and then disable the ones you don't. What if you could Disable it globally and only enable it for the webpages you want. Pretty minor and might not be necessary, but thought it might be a cool option.

3. An option to export settings and webpages you disabled.

also, I did run into a glitch, a webpages stayed dark even when I had it disabled. I removed the Add-on, and the webpage was still dark. I had to Refresh my Firefox, and then it went back to normal, reinstalled the add-ons and disabled it for that page and it was fixed. That's it, good job.