what is “Dead Letters” mostly about?

The main theme in the section entitled, Dead Letters, is coming to grips with loss. Liesel continuously waits for the letter that never comes, and writes her own in order to deal with the immense loss she's suffered.

He says, “You know, Liesel? I nearly wrote you a reply and signed your mother’s name.” He scratches his leg, where the plaster used to be. “But I couldn’t. I couldn’t bring myself.”

Several times, through the remainder of January and the entirety of February 1940, when Liesel searched the mailbox for a reply to her letter, it clearly broke her foster father’s heart. “I’m sorry,” he would tell her. “Not today, huh?” In hindsight, she saw that the whole exercise had been pointless. Had her mother been in a position to do so, she would have already made contact with the foster care people, or directly with the girl, or the Hubermanns. But there had been nothing.