Releases · matreex/vimperator-darkness-theme

If you want firefox to full darkness, maybe you need stylish, greasemonkey, and ft deepdark.

Start Firefox, type "about:config" in address bar, search "xpinstall", will "xpinstall.whitelist.required" to "false".
Download the archive, unzip and will sysle.css replace to ~/common/content in the vimperator.xpi, then install to firefox.

P.S. Firefox 48.x don't install because the add-on must be signed.