I've been growing this Croton in a pot for many years now...Don't fret about the loss of leaves in the winter months. It's quite normal, and they do fill back out very nicely come spring and summer. Mine stays outside unless we are due for freezing temps, and then it comes into the unheated garage for that period of time. Granted, I am a couple of zones warmer than you, but this is really a pretty tough plant.
Good luck! Happy Gardening!
