Assorted tweaks to tropo for foundation

* use _skin-colors.scss from dw-free (shared with other site skins)
* remove unneeded CSS, especially per-page selectors. We can add them in
  later if needed, but most need adjusting to Foundation anyway, so just
  get rid of them and adjust when converting
  (But really, we'll probably find out we don't need them, and end up
  with a more consistent UI all over)
* assorted tweaks to clean up tropo stylesheets
@import foundation into tropo-$color.css

So that we can set per-skin colors on the foundation variables