[Gothika]

The best solution is to have a room above the back room which is a store room that only managers/owner etc can enter. That room would store most of the food/materials with the back room just having enough food/materials. Every couple of days a manager can refil the back room with stock from the store room.

It won't stop stuff being stolen, but atleast it will stop masses ammount of stuff being stolen since they'll only be able to take the limited supplies that are within the back room.