What is the theme of Carl Sandburg's poems?

Carl Sandburg's poem "Chicago" defends the city against dark accusations of prostitution, poverty, and exploitation of the working class. In the end, he suggests that such accusations plague any large city, and he asserts that "Under the terrible burden of destiny," Chicago laughs "as a young man laughs, laughing even as an ignorant fighter laughs who has never lost a battle, bragging and laughing that under his wrist is the pulse, and under his ribs the heart of the people." Thus, he asserts that the spirit of the city not only survives, but thrives in spite of its dark underbelly, and perhaps even has more character because of it. The full text of "Chicago" can be found here: http://www.blackcatpoems.com/s/chicago.html