Well, it's framework-res.apk that you need to modify.
Unfortunately, since it's an .apk (and not a .jar) it's all signed.
You could sign it yourself, but that's not system signed, it won't work.
You could patch something somewhere to not check signing, but that's a whole other thing.
The funny thing is, the signing checks the contents of the .apk (.zip file) but not the file itself.
You can do silly things with renaming the zip local header. That actually breaks extraction of the file. The signing checks through the zip global header so the signing is ok.
See:
https://www.mobileread.com/forums/sh...5&postcount=29
Be very careful because a stock Onyx has a late start ADB and if you break framework-res.apk it will never start ADB!