There are a lot of themes at play in this story, but the central theme it seems to me is one grounded in populism: Underdog vs. The System. The corrupting influence of Capital. The result is families like Toby’s living in near poverty, generation after generation.
Toby decides to break that pattern. He’s not just robbing banks, he’s doing what is — clearly in his mind — right. Maybe not for anybody else, but for he and his family, it’s the right thing.
The whole tone and setting of the story suggests a modern day western, but it’s this theme and that dynamic of a loner doing what is right which really drives home the essence of that genre.