Add verification use of & outside of style rules (#4276)

build/scss/mixins/_backgrounds.scss

@@ -4,7 +4,7 @@
 
 // Background Variant
 @mixin background-variant($name, $color) {
-  &.bg-#{$name} {
+  #{if(&, "&.bg-#{$name}",".bg-#{$name}")} {
     background-color: #{$color} !important;
 
     &,