There are actually two reasons for using white text on a black background. The first, and least worthwhile, is that I happen to like this arrangement. This arrangement is easier on the eyes. The links are not underlined to prevent confusion with things that should be underlined, like book titles or the like. This is opposite the W3's approach, which makes it difficult to underline text in hopes of reducing the likelihood of confusing underlined text with a link.
Active links are red.
This is the standard convention. Visited links are not dark purple, since that does not stand out from the background at all. Instead, they are gray. Neither are unvisited links blue. The text is formatted using sans-serif fonts.