Out of all the cell phones I've owned, the Sidekick has been the only one without the ability to change it's background. Sure, you can choose from 4 SK3 themed backgrounds, but it wont take long for you to tire of Madison, Liquid, Rodeo, and Surface. This instructable with show you how to make and install your own SK3 themes.
Step 1: Making Your Theme
Each Sidekick theme contians a folder comprised of a txt file and various images. Each theme should have two background images, one for the menu screen, and one for the folder screen. Each background should be 240x138px (for creating backgrounds, I recommend using Photoshop or, if unavailable, a free ware program called The Gimp).
Once you have made your two backgrounds, you will need to create your .txt file. Open either notepad or TextEdit and paste the following:
string Name(en_US) theme-name-goes-here
bitmap ring-background file-name-goes-here
bitmap ring-folder-background file-name-goes-here
Just change "file-name-goes-here" to whatever the file name for your backgrounds and you should be set. Don't forget to include the extension! Save the file as "theme.txt" and place into a folder along with the background images.