The law provides that it must be an itemized list of damages, and not just a blanket letter deciding to keep the security deposit. Does the lease say that you can keep the security deposit to cover unpaid rents? If it does, you should appeal for that. Although you didn't mention it at trial, so it is going to be a difficult argument to make.
Often on appeal you have to put up an appeal bond in the amount of the judgement against you, so be aware of that.
There may be other, more technical issues you can appeal. No attorney is going to be able to identify those on this forum. So you might consider talking to an appeals attorney. You should not delay if you decide to appeal. The timelines for appeals are very short.
Best of luck,
Shannon K. McDonald