Keep in mind that theme refers to a central idea or subject discussed in a text that is applicable beyond the plot itself. This novel is of a significant enough length that there are actually several themes discussed.
One prominent theme is the idea of beauty in the face of brutality. At its core, this book is a Holocaust story, therefore, war and death set the tone. The story follows a young girl, Liesel, who manages to overcome the sorror and fear that consumes her by focusing on small beauties in the world.
One example of this comes in her relationship with Max. She describes his hair as "feathers" and visits him in the basement bringing him the crossword puzzles from the paper and weather reports that sound like:
The sky is blue...and there is a big long cloud...like a rope...the sun is like a yellow hole (249).
Despite the brutality all around her (the fact that Max is a Jew in hiding, the fact that Liesel's brother is dead, hunger, fear, and secrecy) she manages to make little things seem more beautiful. The relationship she develops with Max provides multiple opportunities for Liesel to make brutal things beautiful.