This is actually a pretty good theory site, I've used another one created by the same person and it's exactly the same as this one. But anyways, after studying theory in school I went back to this site to review something and now that I know all these stuff already I have to say that it is very good for learning theory and the concepts are explained in a very simple and easy way to understand. Although I would recommend the other one I was using as well because besides the theory articles, you can also practice ear training, so here is the site if you want: