This is an excellent link for discussion of themes:
This is a coming of age story, meaning that it describes growing up. Basically, the themes are the meaning of love, friendship, and the difficulty of being a teenager. There is a good list in that link, with details.
In response to Mahali's Dis.: There are 6 themes... Family Relationships, Friends and Enemies, Generations, Race/Ethnicity/Culture, Challenges and Triumphs, Love/Sex and Romance....