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.