This story has a few possible themes. Perhaps the strongest is the theme of self-identity. After taking the hammer, he acknowledges his wrongdoing, but feels jilted by his treatment when he is caught. He is no criminal, and shouldn't be treated as one. He then struggles with his own view of himself, which has been drastically changed based on his brief foray into the "real world". The entire story is him trying to reconcile the way he was treated with who he believes he really is. Should he fulfill the expectations of the shop owners and delve fully into a life of crime? Or should he struggle to maintain his previous view of himself, and attempt to vindicate his reputation? He settles on a happy medium; he works off the debt to the store, but refuses a job there. He will make up for his crime, but not kow-tow before those who treated him poorly.
Another theme is the concept of ownership. The hammer doesn't belong to him, but he feels he should be able to have it, since he isn't going to do anything wrong with it. Al learns that there are very set rules in the world regarding ownership; not everything is like his mother's garden, where you can take what you need when you want.
For a full discussion of the various themes in the story, refer to the link below. It's a great, in-depth commentary on the story's ideas. I hope that this helps!
The main theme is poverty.
Here is just a brief list:
6. work ethnics
8. personal growth through reflection
If you know the story well, you should be able to find these themes in it. Hopefully this will help