There should be no danger at all - check with the manufacturer for purposes of warranty, but, having worked for many years in medical electronics and as a medical physicist I am sure you have no need for ANY worry.
The attenuation by your body of any fields picked up from the theremin will be almost total (100s of db's) and the signals reaching your body, even if directly coupled without attenuation, to the output electrodes of your pacemaker, would be insignificant - Pacemakers have quite a low impedence output.
Any remote control or demand sensing apperatus used with a pacemaker must meet extremely tight regulations regarding immunity to induced signals.
There is nothing to worry about.. The only grey area is with respect to any legal issues which might arrise if a fault developed on the pacemaker, and the insurer / manufacturer tried to blame the theremin - particularly if the theremin did not have the required certifications (FCC, CE or whatever emmissions regulations are applicable)