There are 2 method to change the KB graphics:
1. Overwrite the original KB graphics,
       however, it will affect all Applications in iPhone, since they all share the same KB graphic files.
2. Build a graphic layer on top of the Keyboard's keys
       however, we found that, we can't press any keys on the keyboard forever

So, the only workable way is build a graphic layer under the Keyboard's keys

Anyway, thanks for every helps