MAGETWO-31855: UI improvements

- variables.less comments adjustments
- popups.less syntax fix

Author: zanilee

lib/web/css/source/lib/popups.less

@@ -245,7 +245,7 @@
     @_popup-actions-toolbar-actions-margin,
     @_popup-actions-toolbar-primary-actions-margin,
     @_popup-actions-toolbar-secondary-actions-margin,
-    @_popup-actions-toolbar-actions-links-margin-top
+    @_popup-actions-toolbar-actions-links-margin-top,
     @_popup-actions-toolbar-primary-actions-links-margin-top,
     @_popup-actions-toolbar-secondary-actions-links-margin-top
 ) when (@_popup-action-toolbar = true) {