It's not the easiest thing in the world to clearly identify where Stein's focus lies. It is mammoth and encompasses so much. One point that the novel addresses is the idea that being "American" is largely a result of human construction. Stein states as much in the exposition of the novel:
It has always seemed to me a rare privilege, this, of being an American, a real American, one whose tradition it has taken scarcely sixty years to create. We need only realise our parents, remember our grandparents and know ourselves and our history is complete.
What it means to "make" an American is largely a result of human construction and Stein uses her novel to talk about how identity, as a whole, is constructed through human actions.
In this, one sees the Modernist tendencies that drive the work. The notion of progressive growth in each generation is one that Stein sees as "dead dead dead." One of the elements that Stein addresses is this idea that future generations don't necessarily improve. The traditional notion is that the next generation will be better than previous ones. Stein speaks of this, repudiating it in her novel. She shows that the subsequent generations are more challenged than previous ones. Stein addresses this in the novel, clearly suggesting that the only legacies that previous generations have bequeathed to younger ones is their "bottom nature" and the more disturbing elements of their psychology. In this, one sees how Stein addresses Modernist consciousness in her novel.