You can still be charged with theft even when you return the item you stole.
Although you returned the item you stole which is good you still did steal the item you returned so you can still be prosecuted and charged with theft.
Returning the stolen item or merchandise or whatever you stole does not get you off on the theft charge unless the person or whoever you stole the stuff from decides to drop the charges.
Returning the item you stole does show good on your part though and can help you in court.
Returning the stolen item can help you negotiate a plea bargain with prosecutors.
But returning the item does not serve as a defense in court for stealing the item in the first place.
The bet thing to do however is not steal anything to begin with.