What is the theme in The Agony and the Ecstasy by Irving Stone?

One of the main themes of The Agony and the Ecstasy is the difficulties of being an artist, in regard to expressing oneself, as opposed to working in a certain medium or theme because that's what puts bread on the table. It's about the ecstasy of doing what you love, as opposed to doing something that doesn't inspire you because it pays the bills.