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.