select2/docs/_sass/vendor/bootstrap/mixins/_alerts.scss

// Alerts

@mixin alert-variant($background, $border, $text-color) {
  background-color: $background;
  border-color: $border;
  color: $text-color;

  hr {
    border-top-color: darken($border, 5%);
  }
  .alert-link {
    color: darken($text-color, 10%);
  }
}