Identity confusion is a big theme. A guy suddenly becoming a girl is a huge shift. He may question who he really is now. Social perception is also a theme. People around him will react differently, and he has to deal with their stares, comments, and sometimes prejudice. Additionally, the theme of adaptation is present. He has to adapt to new clothing styles, new ways of moving, and new social norms associated with being a girl.
The theme of transformation itself is central. It's not just physical but also mental and emotional. There can be a theme of liberation as well. Maybe in his male form, he was restricted in some ways, but as a girl, he feels more liberated. Also, the theme of relationships changes. His relationships with friends, family, and potential romantic interests all shift as he becomes a hot and sexy girl, which can lead to exploring new dynamics in those relationships.