aiiiiiii, You can use either one, but they have different meanings. Grounded on this theory, I can say that = What I am saying is grounded on this theory grounded on ....based on -They grounded their philosophy of life on logic. Grounded in this theory, I can say that = Because I have studied the basics of this theory, I can say that grounded in....having the knowledge of fundamental principles -My classes at the universtiy helped to ground me in the classical ideas of democracy.

The correct preposition is "in." However, the theory would be grounded in something else—something cannot be grounded in a theory ("grounded" means concrete and real, not theoretical). So you can say that the theory of evolution is grounded in Darwin's experiments and observations (but not vice versa).