remove unused pico attributes

[kjaymiller]

picoss is designed to be extremely small and lightwight. The color profiles file (pico.colors.min.css) is about the same size as the primary (pico.min.css) and the recommendation from pico is to remove color profiles that are not used to reduce the size.

It would be good to:

remove any unused colors in pico.colors.min.css add a note about pico and pico's color section in contributing.md