Regardless of the artistic merits of a black background, it has been known for years that white-on-black is very hard to read. Speed and comprehension drop dramatically, and the web site will lose visitors.
There's a reason why virtually ALL commercial web sites use a light-colored background with dark text. They're in it for the money, and they've done the usability studies. Dark backgrounds lose customers. The only exceptions are web sites where the graphical/artistic content outweighs the text, such as a "batman returns" site, or a photographer's site.
Atheist Nexus is not an artistic site; it's primary medium is text, and that text's readability should be maximized.
I hope this helps.