I use NASA Night Launch.
A Stylish style which darkens all web pages.
Edited from Midnight Surfing Alternative
This is pretty good, but it completely trashes some pages, and it's way too extreme. I don't think there's a way to have it not apply to some websites. Disabled for now.
Making the Firefox about:blank page dark.
I think this also applies to about:newtab
A dark theme is less hard on the eyes. I wonder if it lowers power usage for your display.
2016-12-15 - After years and years of having my monitor brightness turned down, I believe this was a major contributor to subtle eye strains which have led to poorer vision. I have turned my brightness up to a power-saving "Eco", and will do so again in a couple of years. Taking breaks is better than turning the overall brightness down. I still prefer dark themes though, but a lot of software is mostly-white and there's little I can conveniently do about that.
- Firefox dark
Simple Openbox themerc -- easily edited to darken your window manager, such as the title of windows.
Inspired by my previous post on Firefox's about:blank, I decided to take some time to investigate changing my desktop background colour.