We would be held accountable for our ignorance, even if someone didn't know stealing is Haram. I'm sure they knew it was wrong? And whatever is wrong is most likely to be Haram. So I really don't see an excuse in the case of stealing. It is not one of those issues that people get themselves into without really knowing the islamic ruling behind it. I.e breaking fast in Ramadan (for a basic example) Stealing is as clear as white and black to be wrong (haram).
But then again everything goes back to what is in our hearts (Which only Allah can know). One should repent and know that Allah is the most merciful. But what was stolen should be given back.
How can anyone not know stealing is bad?
Of course some less informed muslim might do haram things without knowing it is haram. But stealing is such a basic bad things, no one (except maybe a young child) may truly not know that stealing is forbidden.