A theme is a set of resources used to personalize the visual experience on a mobile device. It allows the user to change the look and feel for better user experience without affecting functionality of your device.

Out of numerous themes on a device, only one theme will be activated at a time. Once the theme gets activated, it will change the appearance of your device and of all the apps that have been using standard UI components. The components which are not taken under consideration with active theme will reflect the definition of default theme.

Third party apps that are using device UI components will also be transformed by the current theme. Developers of third party apps should be careful while using components because if they are using both custom and device UI components then there’s high a chance of getting mixed visual appearance of the app.

Note: Third party apps that are using custom UI components will not be changed in any case.

Themes must follow basic principles to provide better user experience. Few of them are;
1. Creativity
2. Usability
3. Media Library

Creativity
A creative theme is the optimal combination of art which is suitable and desirable. It is also a way to express the originality. It doesn’t require many days to create a good theme but with minimal changes according to the situation, you can always come up with some good design.

Courtesy: ChromiPhone theme for Nokia 240x320

Your theme should be attractive, desirable and personal. It is just like a skin that exists on top of default look. It defines something about the person who’s using it.

Mobile User Experience with Theme Design – Part 2
Mobile User Experience with Theme Design – Part 3