Update _background.scss

bg-inverse doesn't exist in BS 4, update to use the new bg-dark

Author: ccoltman

Static_Full_Project_GULP/src/scss/core/utilities/_background.scss

@@ -3,6 +3,6 @@
 .bg-info,
 .bg-warning,
 .bg-danger,
-.bg-inverse {
+.bg-dark {
   color: #fff;
 }