Step 1: Making Your Theme
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.
Step 2: Prep SK3 for Custom Themes
First, plug in your SK3 via usb port on your computer. The sidekick should display as an external memory drive in "my computer"(PC) or in your finder window(mac). If your computer in unresponsive to your SK3, driver installers are available at developer.danger.com.
Once you've opened the SK3 on your computer, you'll want to create a new folder entitled "themes". Once you've created your themes folder, go ahead and drag the theme you made into the folder.
after you've unplugged your SK3, your backgrounds should display along with the four default backgrounds.