Change standard mixers

Hi

I want to change the standard 8x mixer in pixhawk, specifically, change how thrust is distributed across the motors for pitch/roll (right now it seems pitch is achieved by speeding up motors 5+1 more than 3+7). Are there parameters that I can change or is this in the source code?

Thanks!

https://dev.px4.io/en/airframes/adding_a_new_frame.html

and

https://dev.px4.io/en/concept/mixing.html