Find any icons you like from any theme, pull them out of the framework-res.apk or SystemUI.apk (depending on which one your ROM is grabbing them from, mostlikely SystemUI.apk) and drop them into your ROM. Only rule is if your current ROM is using say 100 battery pngs, then you need to replace it with 100 pngs or it will mess up the battery meter.
Its really that simple...