`Color` lacks `Inherit`, `Initial`, and `Unset` instances

[nsaunders]

Mozilla examples:

/* Global values */
color: inherit;
color: initial;
color: unset;

Unlike Clay, which defines its own Color type, this library uses purescript-colors' Color type, which (to my knowledge) doesn't include "inherit", "initial", or "unset" values since these are CSS concepts. So, to fix this issue, I think it will be necessary to define a Color type within purescript-css...