Which boot loader is used on the Sparkfun Arduino Pro Micro boards? Is the source code available so I can modify and recompile it?
I’d like to modify it so PB0/SS (D17) is not used by the RX LED.
Unfortunately this pin is used for SPI SS/CS (chip select) and although you can solder a wire to the RX LED resistor, I’d like to disable any code referencing this PIN so it’s completely available for SPI.
Source should be in our boards package, you can download that at the link below. 
Thanks so much! I think I’ve figured out how to remove the RX and TX LED code.
However, is there another bootloader already written that doesn’t contain the RX/TX LED code? For example, can I flash the Arduino Leonardo bootloader? (I’m not sure if that has those LEDs or not…)
Not that I’m aware of, the Leonardo uses the same method to make it’s RX and TX LEDs work as well. Your fix would most likely work on the Leonardo too.