I've used 'Dark Background and Light Text' [0] on FF for the past couple years. It's been very helpful with eyestrain. (now 48k users)

It will remember one of 4 options (& disabled) for most sites. Choose any color for 6 options (foreground, bg, link, visited, active, selection). Save $$$ when you shop FOSS.

[0] https://addons.mozilla.org/en-US/firefox/addon/dark-backgrou...