Well, a frequent theme is the exploration of gender roles. As a boy turns into a girl, he gets to experience what it's like to be on the other side of the gender divide. This can involve dealing with different social norms, such as how girls are expected to dress or behave. Another common theme is the idea of transformation not just physically but also mentally and emotionally. The character may start to see the world from a different perspective, which can be both challenging and enlightening. For example, in some stories, the transformed boy - now - girl might find that they are treated differently in society, which makes them question the fairness of gender - based treatment.