Evolution is as much a theory as intelligent design
Wrong. Evolution is a theory. Intelligent design is not. A theory is a complex structure of hypotheses with broad explanatory powers capable of making falsifiable predictions and hence having scientific merit. Intelligent design does not fit that description, has no scientific merit whatsoever, is pseudoscience from first to last, and does not deserve to be called a "theory."
A theory by nature CAN'T be proven. It is too broad and complex for that. This is true not only of the theory of evolution but also of the theory of gravity, the theory of relativity, and many other theories in science.
While it is not possible to PROVE a theory (and hence no such proof is to be looked for), it is possible to DISPROVE the theory, and the theory stands accepted until it is disproven or until a better theory is developed. The problem with intelligent design is that it can't be disproven -- no falsifiable predictions can be made on the basis of it -- and therefore it is non-science.