In fem Harry and Dudley fanfic, the theme of growth is quite prevalent. Both characters experience personal growth. Dudley grows in terms of his character, becoming less of a bully and more of a friend. Fem Harry may grow in her confidence or in her ability to handle difficult relationships. Another common theme is the clash of two worlds. Harry's magical world and Dudley's Muggle world collide, and this creates interesting situations and conflicts. It also allows for exploration of how they bridge the gap between these two very different worlds.
Well, transformation is a big theme. Dudley may transform from a bully to a more understanding person because of his interactions with fem Harry. Also, self - discovery is common. Fem Harry might discover new aspects of herself while dealing with Dudley. And family, in a strange way, can be a theme too. Despite their differences, they are sort of family, and the fanfics often explore that relationship in different ways. For example, they might learn to support each other like real family members should.
One common theme is redemption. Often, Dudley is portrayed as having a chance to redeem himself through his relationship with fem Harry. Another theme is acceptance, as Dudley has to accept Harry's femininity and her magical world. And of course, friendship can be a theme as they develop a new bond.