A theme is coming of age. It relates to the setting because the young narrator works in a convenience store and learns a life lesson.
The short story “A & P” is set in a convenience store in the summer. Convenience stores are often staffed by teenagers, and in the summertime customers might wear skimpy clothes. All of this relates to the theme of coming of age. Growing up is hard to do. Young people are not treated with respect by adults, and so Sammy quits his job to protest the treatment of the girls when they are scolded for not covering up. Once he quits, he realizes he is entering the adult world and things will be harder from now on.