Please note that the following information is speculation only and is subject to change.
Themes/Genres are a suggested future feature on Habitica. This would change the characters, flavor text, and images into a different style (for example, cyberpunk or farming) rather than the standard 8-bit theme. This is discussed further in the About Themes Trello card.
Some users have created custom styles/themes to enhance usability, add new elements, or change Habitica's appearance. Players can create their own themes by gamifying the names of their own tasks and of the tasks in challenges they create.