Owl is brilliant at rendering webpages with a dark theme.
I can only give three stars though as I'm experiencing massive slow downs in page loads for sites such as google.co.uk to the point where I nearly went back to Chrome thinking that Firefox Quantum was a bit buggy.
I noticed the slow down after the latest update to Firefox (59.0.1) but I'd also just added the google site to Owls list to darken.
Disabling Owl sped things up again as did removing Google from the list Owl darkens.
So I'm going to try using a mix of Owl and Stylish for making the web shady