Shattered Husky

Update: Shattered Husky has been replaced by Theme Lab: Converter. It's also free and better in just about every way. I am leaving this up just because I don't like the idea of removing stuff from the internet but there really is no reason I can think of to use it.

Shattered Husky allows you to convert any winterboard theme to the IconBundles format. This means you can use a theme built for iPhones on iPad, and across any iOS version.
To convert a bunch of themes at once, open terminal or command prompt, and navigate to the directory of Shattered Husky.jar Then type: java -classpath Husky.jar BatchConverter

    To use Shattered Husky
  1. Copy theme to your computer from /Library/Themes on your device using iFunBox, iExplorer, (in combination with the Cydia tweak "Apple File Conduit '2'") or OpenSSH.
  2. Convert the theme by running Husky.jar. Put the converted theme back on your device in /Library/Themes.
  3. Apply it in Winterboard settings.

Here's a tutorial.