Button Mash is an Input Display written mainly to display SNES controller input.
It currently support SNES Classic hacked with Hakchi2 CE, Arduino based solution for real hardware (using Nintendo Spy firmware), Usb2Snes and regular controllers.
Regular Skin follow Nintendo Spy format so you can reuse skin written for it (see https://github.com/jaburns/NintendoSpy)
The Piano display skins format is Button Mash own, for now you need to look at the default skin provided if you want to write your own
Button Mash project follow the GPL version 3 licence, you can find the full version of the licence on the LICENCE file.