fix(_custom-properties.scss): use same prefix as being used in other partials

Author: larrybotha

scss/core/variables/_custom-properties.scss

@@ -8,4 +8,4 @@
  * Custom properties defined in core can be easily given a different prefix, or use
  * no prefix if this variable is set to ''
  */
-$custom-properties-prefix: s-;
+$custom-properties-prefix: struct-;