fix(layout): provide missing SCSS variables for standalone Layout css

Author: ThomasBurleson

src/core/services/layout/layout.attributes.scss

@@ -10,6 +10,17 @@
 *
 */
 
+// Layout
+// ------------------------------
+
+$baseline-grid:            8px !default;
+$layout-gutter-width:      ($baseline-grid * 2) !default;
+
+$layout-breakpoint-sm:     600px !default;
+$layout-breakpoint-md:     960px !default;
+$layout-breakpoint-lg:     1200px !default;
+
+
 @-moz-document url-prefix() {
   [layout-fill] {
     margin: 0;