What is the theme of the book The Thief Lord by Cornelia Funke?

The major theme of The Thief Lord by Cornelia Funke revolves around the importance of defining family and friendship through love rather than genetics.
The characters Bo and Prosper are brothers who are also friends. They rely upon each other for survival, since their other family members, Aunt Esther and Uncle Max, prove to be unreliable. If Bo and Prosper had not established a friendship with the Thief Lord, Hornet, Riccio, and Mosca at the Star Palace, they would not have survived.
In turn, the children are friends with Scipio, who provides them with stolen items to sell. However, this friendship becomes damaged by the kids' discovery of Scipio's dishonesty: he is not homeless like them but is actually the son of the Star Palace's owner, and he has been stealing items from his own home. This friendship is repaired once the other kids learn of Scipio's ill treatment at the hands of his father; in essence, Scipio has been emotionally abandoned and lives the experience of an orphan.
Victor and Ida are two adults who are not related to these children by blood; however, they establish a genuine friendship with the children by showing them that they actually care about the children's well-being, unlike the children's family members, who don't.
Another theme explored in The Thief Lord is the juxtaposition of childhood with adulthood via the use of the magical, mysterious merry-go-round. What does it mean to grow up or to stay a child? Prosper wishes he were younger so as to be more desirable to the adults that want to adopt his brother, Bo. Scipio wishes to be an adult so as to gain independence from his father. 
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