Fix undefined variable in _settings.scss

ON line 371 the $button-border-color is using a undefined variable $bg I changed it to $button-bg.
Fixes https://github.com/zurb/foundation/issues/5200 :
Child selector added to 'dd' to select only accordion CHILDREN <dd> elements.
Merge branch 'master' of https://github.com/zurb/foundation
small.error and span.error seem to be interchangeable for holding Abide error messages. Added a missing `.error span.error` selector.
Merge branch 'master' of https://github.com/zurb/foundation
Added 'Semantic Markup with CSS' section to Icon Bar Component Page
Change documentation of button mixin to be consistent with the other componeents
Merge pull request #5390 from jameswlane/patch-1

Fix undefined variable in _settings.scss
Merge pull request #5402 from cautionbug/master

Accordion 'dd' overreach on active change
change  div.icon-bar to be instead .icon-bar in order for the component to be used with more semantically meaningful elements
Revise the Magellan script to highlight the element(s) of a particular destination after a click or tap instead of after a scroll
Final versions of Using-Sass and Sass-Files pages
changes
QA Fixes
Fix some headers and hr tags