Jeff has said that the site is about programming, instead of Computer Science in general. Although Computer Science related topics are often discussed anyway. I cannot say for sure, but I think he specifically said programming so that people didn't try to include questions like hardware setup. And not to exclude theoretical Computer Science topics.
I do think that you can ask your questions here, even if it is about theoretical Computer Science and it will be well received if it is a good question. I know at least I would upvote good theoretical Computer Science questions. I have seen many past theoretical CS questions highly voted up.
It comes down to the fact that there are many capable people on this site that can probably help you with your theoretical computer science questions. And if a lot of these people like your question, then as a community it will be voted up and answered. I can't see a moderator deleting a question that is well received by the community.
Other source of info:
Other than this site, you could ask for recommendations for the topic you want to know about from professors in your Computer Science department. Or to a school that is local to where you live. Also, depending on what you're studying, a lot of times the classes (that cover your topic) in University have a forum per class.