So I came across this early on in my research of theme porting, but was still learning and didn't fully understand the process. However, now that I have some experience under my belt I decided to take a look at it and see how it worked.
Basically by use of this app we can apply created .zip packages that contain themed image's for any .apk located in /system/. In short, we can theme individual pieces of our phone to our taste, by applying a specific .zip.
There is a very specific process to doing this, because there are certain requirements to inserting .png files into your phone. The files you have flashed may have changed XML values that point to specific images, if that XML points to 20 images then the .zip you create must contain different versions of those specific 20 images - in order for this to work properly. In other words, the images already have to be called in the code for this to work, so we have to be careful how we build these .zip's so they work right.
Basically what we need to do is just start creating addon packs for our existing theme's, that way we know it will work as long as that theme is installed (it has the XML to support the .png file addon packs)
That means, you get your little tweaks without us having to create entire new theme's or having to bother at all with the long process of making these edits, we just have to find the .png's you want changed and include them in the package.
Watch this video; http://www.youtube.com/watch?v=p9y3ooUZZrg
I've tested this once on just my battery (because the accurate battery xml is universal, it references 100~ images) I used the battery from my recent port A Splash of Color, i pulled the .png files of just the battery icon and transplanted them to the .zip i created (watch the video, it shows you how to do it) Pushed the .zip to the AndroidThemes directory on my phone (that the Metamorph app created) then loaded up the app. In the app, my test.zip showed up and everything worked great - at the end the phone vibrated as if it was force closing but it was just rebooting, when it was done the battery icon was changed, with no need to wipe dalvik or use recovery at all.
Go ahead and try for your self, here is my testing.zip (you must have at least pick and pack and a theme with the accurate battery mod installed for this to work);
I have my EA11 dark froyo port that i'm working on, and you'll see i've changed the battery to the EA11 Splash of Color battery
Then stay tuned for some updates on the theme's i've ported