A theme packages up styling code into a single unit that may be used to modify the appearance of all UI components in the same project. One advantage of themes is that they keep styling code from getting mixed up with things like business logic and gameplay mechanics, which helps make your projects more maintainable. They also make it easier to share styles among multiple related apps or to swap out the styles when a project needs a fresh coat of paint.
Feathers UI provides a default theme, named Steel, which provides basic styles to all of the core UI components. A custom theme may be created to replace the styles from the default theme.