What is the theme in Extremely Loud and Incredibly Close by Jonathan Safran Foer?

Extremely Loud and Incredibly Close is filled with death, because the primary narrator, precocious nine-year-old Oskar Schell, is obsessed with it since his Dad perishes in the collapse of the World Trade Center, 11 Sep. 2001. Oskar needs to know which of the many forms of death is Dad's and accepts that death is inevitable for every living creature. When he cannot sleep or is anxious, Oskar invents protective devices to ward off violent death and scrupulously avoids "obvious targets."