One common theme could be friendship turned into something more. Maybe they start as friends and gradually develop romantic feelings. Another theme might be overcoming obstacles together, like dealing with outside forces that try to keep them apart.
Common themes in 'Humphrey x Lily Lemon' fanfiction are quite diverse. Romance is, of course, a big one, with all the typical elements like first kisses, declarations of love, and jealousy. Family can also be a theme, either their own families interfering or them creating a new family - like bond. Another theme is the exploration of different worlds or settings. Maybe they travel to a magical place or a new city, and this new environment affects their relationship. There's also the theme of sacrifice, where one or both of them might have to give up something for the sake of their relationship.
In 'Humphrey x Lily Lemon' fanfiction, a frequent theme is self - discovery. As they interact, both characters might learn more about themselves. Also, the theme of redemption can be present. If one of the characters has made mistakes in the past, the relationship with the other can be a path to redemption. Additionally, the theme of adventure often pops up, where they embark on exciting journeys together and their relationship grows during those experiences.