Can I be held financially liable for damage done to other cars if my car was stolen?
Usually you would not be liable if your car was stolen: people are only responsible if they are either at fault themselves or assumed responsibility, either explicitly or by action of law (the way the law makes parents responsible for their minor children).
However, there are circumstances under which you could be liable:
1) You were negligent and "allowed" the car to be stolen--parked it or left in a bad area with keys in the ignition and/or running; you have a roommate who has stolen cars before but you left the keys out where he could find them; you had some specific warning (someone had said they were going to take your car), but took no protective steps; etc.
2) You are responsible for the actions of the thief--for example, he or she is your minor child, or some other child for whom you are the guardian