src/scss/bootstrap-4.0.0-alpha.6/scss/mixins/_background-variant.scss

   1 // Contextual backgrounds
   2
   3 @mixin bg-variant($parent, $color) {
   4   #{$parent} {
   5     background-color: $color !important;
   6   }
   7   a#{$parent} {
   8     @include hover-focus {
   9       background-color: darken($color, 10%) !important;
  10     }
  11   }
  12 }