Sorry to bring up an old thread, but i was trying to find or even make my own green battery header files so that the icon's are green even at the home screen when it is usually white, sadly it took multiple tries to understand how easy it actually was, since the default icons with a white background are green all you need to do is rename them. Download the stock images here:
iPhone Forums at ModMyI.com - iPhone | iPod Touch, news, apps, themes. - Downloads - Stock iPhone Images COVERTED
in the system/library/coreservices/springboard.app folder
then copy the green ones (default ones) and rename them to their respective FSO version here is the list of all the files you will want to copy then rename
Header Battery
-System/Library/CoreServices/Springboard.app/Default_BatteryBG.png
-System/Library/CoreServices/Springboard.app/Default_BatteryCharged.png
-System/Library/CoreServices/Springboard.app/Default_BatteryCharging.png
-System/Library/CoreServices/Springboard.app/Default_BatteryInsides.png
-System/Library/CoreServices/Springboard.app/Default_BatteryLowBG.png
-System/Library/CoreServices/Springboard.app/FSO_BatteryBG.png
-System/Library/CoreServices/Springboard.app/FSO_BatteryCharged.png
-System/Library/CoreServices/Springboard.app/FSO_BatteryCharging.png
-System/Library/CoreServices/Springboard.app/FSO_BatteryInsides.png
-System/Library/CoreServices/Springboard.app/FSO_BatteryLowBG.png
hope that helps, i added a screen shot of how mine looks with the changes. also if its allowed i can zip or rar the ones i have and upload them here, wasn't sure if that was kosher or not.