Well first it's kind of a nonsense question. Admins are typically the only ones changing pages (themes or permissions), so you're trying to audit what your admins are doing. If you can't trust your admins, well then I think you have bigger issues. Users can change their own pages themes and permissions, but as it's their own pages auditing that information is pretty darn pointless.

Now auditing in general is not all that difficult. In fact, when you use the audit plugin from the marketplace, you can see that they use ModelListeners to intercept calls for the Users table. Using that model, you can create model listeners for the various classes in order to add auditing information.