Depends on how you're using the term 'Theory'. The layman's concept of a theory is basically that it's little more than an educated guess, and that almost any theory is just as valid as any other. But in scientific terms (and yes, economics is a science, if not a rigid one) a theory is a model of reality. It explains what can be shown to be true, and predicts new facts which may not yet be understood.
The problem with economic theories as I see them is that they rely on the actions of people, which is a very difficult thing to model. And in the modern world, with almost instantaneous communications and global markets, the problems are multiplied exponentially. When you add in government interference, the results become completely unpredictable. The models might work, on a small scale, but be completely useless once government regulations alter the playing field.