What deep learnng theory we really need?

When we talking about the deep learning theory, what theory we really need? We also say that deep learning is a blackbox, we do not have a solid theory for deep learning. But what kind of deep learning theory you think we really need to have. If it is just principle of modern neural network, I think there are already such theory like universal approximation theorem, and backpropagation automatical derivation theory.

So what kind of aspect you think we need …

blackbox deep learning deeplearning kind modern solid theory think