A theme in any major work of literature is a central idea in the story. Put a different way, themes are the major subjects discussed through the plot of a text which can be further discussed as deeper truths spoken by the author, thought about as lessons that are taught, or compared to real life. It is common to mistake the idea of "themes" with "plot summary" and important to keep in mind that theme subjects are enhanced by the plot, but go much deeper than the basics of the story-line.
The Book Thief is set in Germany during World War 2, and it offers a different perspective of the Holocaust by relaying events through the eyes of "Death" as a character, and focusing on a young orphan girl who is not Jewish, but has lost both of her parents because of allegations of communism.
One of the most obvious theme subjects, therefore, is death. Every character in the story is personally affected by death, a natural theme of a war story. However, because this book is told from the perspective of the Angel of Death, there is a juxtaposition of death as a terrible and horrific consequence of war, and death as a perfectly normal and natural consequence of war, and life in general. Death, the narrator, tells this story of a horrific time in history in a straightforward and almost emotionless manner. It can be argued that the author's purpose in this approach to a fairly common theme, is to actually heighten the horror of the Holocaust by forcing readers to react without contrived emotion.
In addition to the theme of death, The Book Thief is a book about unconventional and nontraditional friendship. Liesel is of course orphaned at a young age, pulled from her home and family, and left with a profound loneliness heightened by fear and confusion. The plot of the story takes readers into three very important and very unconventional relationships.
The first is the relationship between Liesel and Hans Huberman, the unconventional father. Absolutely nothing about Hans Huberman's approach to fatherhood is traditional nor expected, yet he alone earns the kind of love and trust that a young girl reserves for her daddy. The second is her relationship with Rudy Steiner, the unconventional first love. Again, there is nothing traditional nor expected about Liesel's relationship with Rudy, who every day asks for and fails to receive a kiss from her. However, upon his death, it is obvious that Liesel does in fact love him, and may never love anyone like this again. And finally, Liesel's relationship with Max Vandenburg is important. This one is so unconventional that it is almost impossible to define. Max, a Jewish man in hiding, is the first person to connect with Liesel in what may be the most personal way. Using the fewest words and least amount of time, it seems Max understands Liesel on the deepest level.
Through these, and other unexpected friendships in the book, the author shows that self awareness and identity are often defined in unlikely ways.