* Remove the 'line-height' property from .dropdown-menu-btn that,
   combined with default top/bottom padding for <button> elements
   was causing the button label to be vertically off-center in
   the 'Post to' button.

 * Remove duplication between .primary-action-btn and .dropdown-menu-btn

 * Use $color- variables for palette consistency in .dropdown-menu-btn

T-125