A visual motif for a user interface.
A theme or skin includes a set of matching elements that change the look and/or sound of the user interface, including the home screen, background image, icons, menus, startup graphics and sounds, etc.
Phones with theme and/or skin capability may come with several options pre-loaded, and often allow more to be downloaded and applied later.
A theme / skin may have a generic theme, such as "tech" or "nature", or a something more specific, such as a specific sports team.
Themes may also include sounds, such as ringtones, on/off sounds, key tones, etc.
Some phones have separate "skin" and "theme" functions, where the skin controls the look of the menus, and the theme includes other elements.