You cannot be charged with the theft since the owner of the key voluntarily gave them to you. However, there are consequences that you would face by failing to return the keys. You can find more information on the matter from a lawyer.
Give the keys back. If u are refusing it is a crime. A lawyer isin't going to help you. Just because the person gave you the keys doesn't mean you have the legal right to keep them. Give the keys back. You're setting yourself up for trouble.
Sadly, this can fall under several classifications.It can be seen as joy riding, to grand theft auto, all because of the different laws pertaining to the use of a vehicle in each different state. Most will see the giving of the keys as a loan, and not returning the car as theft.
If the owner asks for the return of the vehicle then you must return it or else it is theft, unless you have the car registered in your name then you really shouldnt be driving it. If you have damaged the vehicle in anyway you you must pay for all repairs or compensate the owner the amount for repairs. Lending a car to some one doesnt mean you can use it forever, just a reasonable time.