As a wannabe computer "scientist" I have problems differentiating theory and practice
There is no universal objective answer to the question of how to tell what counts as theory and what counts as practice, or what counts as science vs engineering; the precise dividing line between theory and practice, or science and engineering, is subjective and a matter of opinion.  Those terms are not precisely defined -- they are fuzzy, subjective terms.  Different people use them differently.

For instance, sometimes people might use science to mean "understanding fundamental questions, identifying and testing laws of nature and hypotheses" and engineering to mean "building useful systems and solving practical problems", but even there the line is fuzzy.  Other times people use the term "science" to mean "what I do" or "the kind of work I respect" or "the really challenging stuff" and "engineering" to mean "what those other researchers do" or "the kind of work that I consider trivial/uninteresting/not deep".  You'll probably encounter many other variations on these kinds of meanings.

P.S. You are wrong that it's hard to do research with experiments in HCI.  Indeed, every human subjects study is itself an experiment, so experiments are very commonly used in HCI.  Perhaps you are thinking that it is challenging to find universal laws of nature in HCI (analogous to Newton's First Law, say, but for people), as people are complex.  That is true, but it doesn't mean no such laws exist or that such research is impossible.  Consider Fitt's Law, for instance.