The Theory plug-in enables modellers to extend the mathematical modelling notation for Event-B, with accompanying support for reasoning about the extended language. Previous version of the Theory plug-in has been implemented based on Rodin 2.x. This presentation outline the main improvements to the The- ory plug-in, to be compatible with Rodin 3.x, in terms of both reliability and us- ability. We will also present the changes that were needed in the Rodin core to accommodate the Theory plug-in. Finally, we identify future enhancements and research directions for the Theory plug-in.