The themes of Death of a Salesman include:
A) Willy's quest for the ideal of the American Dream and the idea that those who work the hardest get rewarded the hardest. His idealized notion of the All American "perfect" life with the son in a football team, him a businessman with a wife, even a mistress- and the fantasy of it all.
b) reality vs. fiction- All that Willy had as real was actually his own make-belief notion of grandeur. Back in his time, a salesman was probably the least educated professional of his time, yet, Willy saw himself as a major businessman the way self made millionaires would see themselves today. The dysfuctionality of his family, his lack of parenting skills, his torn marriage, his insipid career, all this goes in the backburner in his mind.
c) fighting against society- A salesman has no choice but to codepend on circumstances: The market, the clients, the trends, the business, etc- Willy tried his entire life to build something he could fall back on with no success. He was as incapable of building a present as he was a future.
d) fighting against oneself- Willy had denied his talents, froze his son's own talents (though Biff was no different than Willy) and all because he still wanted to live this image that he was not up to par. In the end, he died committing suicide, perhaps after finally accepting how little he had accomplished versus how much he had dreamt.