Why not reflashing the rom ?
It's pretty much impossible to do that. IF you choose correct mod
SystemUI can have diffrent mods yes, like 1% battery, 23T or stock, stuff like that. (So make sure that match with what you have atm)
So you should not download a 1% battery theme if SystemUI don't have it - and vice versa.
Also, you said you didn't want anything else, don't worry - it's impossible since battery is SystemUI.apk and the examples you gave
is secphone.apk, android.policy.jar, so you can never get anything more with just a batterytheme.
Other than that battery is only icons, so it can't mess anything else up - as long as you download correct!
And now answer to your problem, you have the rom zip, right? In there you have /system/app/SystemUI.apk , now you want that SystemUI to your phone, in same path /system/app/
Fastest way is to go to your file on compter and with phone hooked up with USB, 'adb push SystemUI.apk /system/app/' , if you don't have that then put the file on phone somewhere, then with rootexplorer or similar, put the file in correct place and reboot.
With that you can ALWAYS restore phone without reflash rom, so learn a method that works for you. And next time, download a SystemUI theme that matches what you have now. Problem solved.
Very handy if you want to go back in any way, like SecSettings.apk, everything can be done this way. That's how we try new features and so on, before a release. We don't reflash rom 1,000 times, but we 'adb push' _a lot_
Might have beem confusing, but it's just ask again if anyhins is unclear.