iPXE general.h

Hi,

I'm a novice Linux user, and I'm trying to setup an iPXE server on Ubuntu.
Everything is working fine, excepted that I didn't find the way to change the way my menu is showing up (text color and placement, background)

The only thing I could find about it, that I'm probably missing is the general.h that needs to be added. But I do not understand where it should be stored on the system and how I should apply the changes I need to it.

My goal is to put a PNG image file as background and make sure the text color is white.
Highlighted text should be set too, so it remains in contrast with the background.

I found the commands to do it, but do not know and didn't find out how to enable those.

Please, help !
Thanks in advance