Has anyone ever successfully stolen a commercial airliner in mid flight?

No. All aircraft are fitted with electronic tracking devices that automatically report their position. The flight-path the plane is expected to take has already been logged with air traffic control. Any hijacked plane would be instantly spotted the moment it deviated from its logged flight-path or altitude. It's impossible for someone to simply walk onto an airports airside, get into a plane and fly it away - without someone knowing !