A theme is the complete skin of EmulationStation: background, system carousel, game list layout, fonts, sounds, animations. Since Recalbox 10 everything happens on the machine itself, no computer needed, thanks to the theme manager.
The topic is covered in detail in a dedicated section:
/recalbox/share/themes.To create or adapt a theme, head over to the format documentation: 🎨 Themes.
Built a theme? Show it on the Recalbox Discord: that is how a theme joins the official repository.