It isnt an app that u can just theme with so easily.
U have to open apktool using "CMD" or Command Prompt
If u want to theme easily use ticklefish's Tickle my droid
If u want to theme as u want exactly use apktool.
Also If u just want to edit images (and not xmls)
Just use 7 zip to extract ur apk. Edit pngs in drawable-ldpi and drawable-hdpi using gimp/Paint.net/Photoshop
then put edited files back in the apk
Real Men(Contributors) Dont Need To Ask For Thanks. They will get it For a Job well Done
(Not using Nexus S. Just a build prop edit on my star.)