[pfe-button]: color context design spec

[brianferry]

Description of the issue

The pfe-button component does not switch colors when the parent has a color-palette of "darker" or "darkest". This is in contrast to the guidelines on Red Hat UX's site and we will want to revisit whether or not this is expected behavior for the patternfly elements button components.

[brianferry]

As per our discussion we do not think that we need to mimic the Red Hat design doc completely but want to make sure that what is there is the expected behavior.

[bennypowers]

#1976 partially addresses this, but we need some design direction. When that PR is merged, let's rename this issue to [pfe-button]: color context design spec or some such

cc @markcaron

[bennypowers]

See also #1997 and #2014