_single.scss

.select2-container--bootstrap4 {
  .select2-selection--single {

    // height of input box
    height: $input-height !important; // stylelint-disable-line declaration-no-important

    // placeholder
    .select2-selection__placeholder {
      line-height: $input-height-inner;
      color: $input-placeholder-color;
    }

    // arrow
    .select2-selection__arrow {
      position: absolute;
      top: 50%;
      right: 3px;
      width: 20px;
      b {
        position: absolute;
        top: 60%;
        left: 50%;
        width: 0;
        height: 0;
        margin-top: -2px;
        margin-left: -4px;
        border-color: $dark transparent transparent transparent;
        border-style: solid;
        border-width: 5px 4px 0;
      }
    }

    // selected item
    .select2-selection__rendered {
      padding-left: $input-padding-x;
      line-height: $input-height-inner;
      color: $input-color;
    }
  }
}

// search box
.select2-search--dropdown .select2-search__field {
  padding: $input-padding-y $input-padding-x;
  border: $input-border-width solid $input-border-color;
  @include border-radius($input-border-radius);
}

// no result message
.select2-results__message {
  color: $secondary;
}