Themes (skins) are used to change the look of an individual's mobile phone user interface (UI).Traditionally, themes are used to alter frames, colour or font size in the PC environment. On a more limited scale, you can also use themes to modify the entire look of the graphical user interface.

First generation themes enable changes to the basic UI look. Themes allow you to:

Change the display background 
Change the look of highlights 
Change the look of pop-up windows 
Modify the look of commonly used components 
Additionally, themes may contain items specific to individual applications, such as the background image for the idle mode and customized icons.

It's also possible to embed a ringtone in a theme file of a phone handset, (e.g. most Motorola handsets). Thus, it's possible to say, send a phone theme file of a CD cover, together with a ringtone of one of the songs in the CD in just a single file.

A single theme package can contain graphics for one or many changeable components. If a theme package does not contain a new component, the default UI graphics are shown.

Changing a theme only affects the look and feel of the UI. The functions or features of the device are not changed.