I believe one of the themes in The Book Thief is bravery. This is demonstrated when Liesl purposely scraped her knee so it wouldn't look suspicious when she ran into the house to tell Mama that the soldiers were there to check basements. Another time this is shown when Liesl shouted "I hate Hitler!" when she was at the lake. Bravery was shown again when Liesl was stealing books. All of these were acts of bravery because if she would've been caught, she could' have been killed.