In any literary work you are going to encounter a number of possible themes, so I will just respond by talking about one. I have included the link below to the enotes section on the themes of this work, so that should help you out with identifying others.
One clear theme in this interesting play is the relationship between good and evil and how in particular Shaw presents traditional notions of these concepts as problematic. This is shown perhaps most clearly in Barbara and Undershaft. Barbara, who has sacrificed her life to working with those in need, is clearly a "good" character. She is presented in contrast with Undershaft, who is wealthy because of his unscrupulous policy of selling armaments to anyone, regardless of their cause or values, clearly being an "evil" character.
However, note how these clear, black and white distinctions become blurred into a disturbing grey in the second act when we discover that the Salvation Army receives donations from both the armaments firm of Undershaft and an alcohol business, which of course causes many of the problems of the poor people that they deal with. Likewise we see that the workers in Undershaft's "evil" factory are given very good accommodation to live in. His business has allowed Undershaft to give his workers a good standard of living, ironically being successful precisely where the Salvation Army has failed. In a sense, the play is all about the journey of Barbara towards seeing the interrelatedness of the concepts of good and evil and how truly messy they are. She leaves the Salvation Army because she cannot understand this, but then at the end of the play we see her becoming more aware of the complexities of these apparently simple concepts.