I would avoid this addon. While it works very well, as advertised, the developer charges money for it after a 3 month trial. That would have been ok with me, except it's a subscription-based model. Very ridiculous. I would have been ok with paying maybe $5 (which is fair for what it is) for an outright lifetime license. But anyways, I recommend for something similar, try using the addon called 'Dark Night Mode'. It's similar and works well out the box, has a higher rating, and is fully free. For something with more features and customization, I really like 'Dark Reader'.