They use the approval of the GRE that teaches the courses. As I said right at the start you can do the theory which whoever you want. There is a guy called Derek Davidson with an great reputation who specialises in PPL ground school.
However as I also said, ensure that your school is happy with it. As a rule we prefer that our students do the theory with us and many other schools do the same. We do get a few that turn up with the theory passes already signed off which we accept but if you are doing the course from zero with us then we insist that includes the theory.