What is the specific theme being expressed in the play "Pygmalion"?
At the risk of sounding like Bill Clinton, what specifically do you mean by the word "specific"? (If you're too young to get that joke, check out the link below.)
Unless our name is George Bernard Shaw, we can only attempt to say what is the theme of "Pygmalion."
In my humble opinion, there are two main themes in "Pygmalion."
a) Distinctions of class (upper class, lower class, etc.) are artificial. If a "guttersnipe" like Liza Doolittle can be trained to the point that she can be the star of a high-society ball, then what real difference is there between the guttersnipe and the princess?
b) Education can modify our behavior and our attitudes, but only up to a point. At the end of the play, Liza chooses to marry Freddy and run a flower shop rather than live the high-life with Higgins. She would be utterly incapable of returning to the gutter--but she is just as incapable of dealing full-time with Higgins's snobbery, intellectualism, and occassional forays into high society.
Remember: theme is a matter of opinion. Your opinion must be reasonable and well-supported by the text, but it is still your opinion.