Multiple themes are seen in "The Book Thief" but the main theme that stands out is that "words are life." If it was not for the books that had not been for the books Liesel obtained, the entire relationship between her and others would have changed.
In the book, Liesel finds Rudy's dead body and finally kisses him. This is very significant because Rudy was always asking her for a kiss, although he knew she would refuse. Rudy was in love with Liesel and Liesel was secretly in love with him too.