What is the theme in When I Was Puerto Rican?

The main theme of the story is identity. Throughout the book, Negi struggles to find a stable cultural identity which will allow her to live in the United States while still retaining her Puerto Rican heritage. This struggle is so intense that Negi's complex, ever-shifting identity manifests itself in a number of out-of-body experiences, which illustrates just how deeply conflicted she is with herself.
Whenever Negi looks inside herself, she finds deep rifts in her cultural and ethnic identity. In turn, those rifts are reflected in the social world outside. As well as the obvious conflict between American and Puerto Rican identity, there's also an even deeper conflict within Puerto Rican society itself. In pursuit of an identity of her own, Negi adopts that of the jíbaros. The jíbaros are simple country folk renowned throughout Puerto Rico for their art and music, yet they're also looked down on by Puerto Rican city-dwellers for their relative lack of sophistication. Negi's mother tells her in no uncertain terms that adopting the identity of a jíbaro is completely inappropriate for someone of her background. In due course, Negi comes to learn from this episode and realizes that identity is not something you can simply choose; it's a good deal more complicated than that.