I have come across the Dark Reader extension for Firefox:
https://addons.mozilla.org/en-US/firefox/addon/darkreader/
One nice thing about it, in addition to all the display controls it has, it also has the ability to override fonts on a global or per-site basis.. so on Gmail on the extension's "More" tab you can change the font to your browser's sans-serif, etc. instead of what Google chose for you.
I think you will like it.. play around with the settings it has.