As of version 2.2 of Badass Battery Monitor (all versions), it's now possible to create third-party icon packs that will work with Badass Battery Monitor.
See the following link for details as to how to do it. Instructions are provided in the wiki, and the source tab of course contains all the source for a complete Eclipse Android project. It's fairly simple to do - very little modification is needed - with the exception of the icon sets of course!
You're free to create your own icon packs for your personal use, or make your creations available in the Market or Appstore (for free of for $$$ - up to you).
Here's the wiki showing you how to do it:
You can see available themes in the market using the following URL: