twbs--bootstrap/scss/mixins/_background-variant.scss

// Contextual backgrounds

@mixin bg-variant($parent, $color) {
  #{$parent} {
    color: #fff !important;
    background-color: $color !important;
  }
  a#{$parent} {
    @include hover-focus {
      background-color: darken($color, 10%) !important;
    }
  }
}