by keypad im going to assume you mean home screen with all the icons?
you can read information on how to skin for windows
here
if you want more information on skinning and need help with graphics
here
the graphics you want to try to modify are:
DefaultDialer.png
dialerkeypad.png
dailerkeypad_pressed.png
you can find the original files
here, pull them into photoshop, modify them, then upload them back on to your phone.
edit: didnt see this thread until just now, go
here for help on dialerpad/howto