One of the common themes is the power of emotions. Since Ahiru is a very emotional character and Mytho is somewhat learning about emotions, their fics often deal with how emotions can drive actions and decisions. Another theme is the exploration of different worlds or dimensions. Some fanfictions might take them on adventures to other places, and this exploration can be a theme in itself. And redemption can also be a theme, especially if there are past wrongs or mistakes that need to be made up for in their relationship.
Love is a very common theme. Their relationship, whether it's a budding romance or a deep, established love, is often explored. Another theme could be self - discovery. As they interact, both Mytho and Ahiru might learn more about themselves. Growth is also a theme, as they face challenges together and grow as individuals and as a couple.
Well, transformation is quite common. Given the nature of the characters, there could be stories where they transform in some way, either physically or in terms of their personalities, and how that affects their relationship. Also, the theme of sacrifice might come up. They might have to sacrifice something for the sake of their relationship or to protect each other. And, of course, the theme of friendship is often intertwined with their romantic relationship, showing how their friendship forms the basis of their deeper connection.