Higher contrast output

[Kernald]

Currently, printing the output isn't that easy to read from a distance - the keys have a fairly dark grey background, making that a fairly low contrast to the labels in black. Unfortunately, --transparent only changes the actual background of the image, not the keys. I would love an option to change the colourscheme to have white keys, or at the very least a way to have a higher contrast ratio between background and foreground (bonus points if it doesn't use colours - I only have a black and white printer...)