Oh, it's not a thief. If it was, they wouldn't leave such obvious evidence. When you frame someone, you want to make it look like they're the ones who accidentally left something at the scene of the crime, right? Light blue in this place is really easy to find. Especially when it's just out in the open, not too much dirt on it, and only old stains to show.
My guess is? It's an amateur. Not even a thief. Just someone out for petty revenge.