Then what is theft?
I'm glad you like the painting, but does looking at it give you pleasure when you know it doesn't belong to you? Or is your conscience so calloused that it doesn't bother you? If that's the case, then I find it curious that a painting could move you enough that you'd be willing to steal it.
Or maybe it's a Christmas gift that you couldn't afford. Either way, what you did is something you will have to deal with yourself. I refuse to worry about it. Christmas is all about a gift that nobody deserved, so merry Christmas to you, and I sincerely hope you enjoy the painting.