The theme is the central idea or an insight about life that the playwright wishes to express . what is the central theme of all my sons ?

There are a few major themes to this play. Personally I would narrow them down to "The American Dream" and "Denial and self Deception". Check out this GrsdeSaver link and you will see the major themes including the one's I mentioned.

http://www.gradesaver.com/all-my-sons/study-guide/major-themes/