While it is nice, there are more lightweight and snappy ways to achieve a dark background (or desirable styles in general) -- such as setting a global CSS, using built-in color overriding (at least in FF it's available, along with font overriding), using web browsers that don't apply CSS. Some people even use system-wide color inversion. Each method has its pros and cons though.

Stylus is one of the handy FF extensions, which allows to switch between CSS themes quickly/easily.