Stylish is a Firefox Add-on that lets you install and manage user styles. User styles exist mainly to change the way things look. They are written with CSS, the same language that web sites use to control their appearance. This addon can be installed from https://addons.mozilla.org/en-US/firefox/addon/2108
I feel more comfortable looking at black/dark backgrounds. So I have installed dark styles for many frequently used sites. Can I get one such Dark color scheme customized for superuser.com web pages. Example there is a of Black color scheme for google web pages is here: http://userstyles.org/styles/1693