What is the theme of "Thank You, Ma'am?"

To me, the theme of this story is that people can be transformed by love and trust more than they can be transformed by fear and punishment.

In the story, Roger is a thief.  But when he meets up with Mrs. Jones, his life is changed because she catches him.

But she does not call the police or tell his parents or anything.  Instead, she treats him well and shows that she trusts him.

This works, in my opinion, because people often behave the way they are expected to behave.  That is the theme of the story in my opinion -- the idea that if you trust someone and treat them well, you can change their life.