On the theory side of things there are the 5 Taijiquan Classics. To understand these short texts requires some cosmological background informed by Confucian thought and Daoist classics, most notably the Huainanzi.
But the Taijiquan Classics are mostly just lists of what to do or what not to do to achieve a somewhat elusive set of goals. Sure, to understand these lists you need to flush out the various metaphors used: Landscape, purification, water, pearls, coins on a string, a scale, following a compass, etc. But still, we are in essence dealing with a list of do's and don'ts, more indicative of a method than a theory.
After all, what's the goal again? To be weak? To be the greatest fighter on this side of the Golden Gate Bridge? To make clear commitments? To feel beautiful? To be so sensitive and intimate with your opponents that you know them deeply, but they can never know you? That's some weird stuff.
Oh yeah, and long life. Sounds good, but there isn't much theory there.
Some might argue that wuwei, non-aggression, is the theory. But I would say this: Taijiquan is an information storage system. It is a whole bunch of ideas, some of which fit well together, and some of which strain the boundaries of what can even be communicated between two people. For the most part these ideas are experiments which are meant to have some discrete result (which may or may not be part of a larger idea). So? Do the experiments and see if they are true. If they don't workout, discard them or, if you are a lineage holder, put them back in storage. That is the formula pure and simple. That's the only way it works.
Taijiquan is an experiment you do, on your own time! People who just go to a Taijiquan class a few times a week never actually learn. It is not something that can be spoon fed.