The usability of a web site is one of my pet subjects ..the navigation has to be intuative, the content has to be delivered in a logical user friendly way and a site has to load quickly …just a few of the issues thyat affect the usability of a site.
Another is the readabilty of the content. Compare the following 2 paragraphs …which is the more readable?
Using a dark background with light text can be an effective way of drawing the users attention to an area and in the example above this is the case. However defining a whole web site to have a dark background and light text can be very tiring on the eyes and make the content difficult to read.
It is also important to have adequate contrast between the the text and background.