What is this StandardFirmata_ModifiedAll code doing?

The sketch doesn't even declare TOTAL_PINS, but it runs well, what happen?

Sure it does. It includes Firmata.h which includes Boards.h which defines a number of platform-specific values (based on the board type).

So what you mean, I should do try and error?

I guess. I don't even know what changes you want to make to what GUI.