Stealing is an action. I think it should be based on weight of the armor you are wearing, the blending of the crowds and, how often you do it(skill). Another thing is the target storage location. example a small pouch of coins hanging from a belt or a grabbing bag from a caravan. Less you carry, Harder you are to steal from. It makes sense. Stealing a dagger from a assassin would be harder based on where is located. if it is in his boot, I wish you luck. Stealing a pouch of coins from a tank wondering in town would be much easier.
I was looking through the Archetypes and found myself looking at the Ranger class. I wondered how they would fit in with the other classes