grafana/public/sass/components/_form_select_box.scss

$select-input-height: 35px;
$select-menu-max-height: 300px;
$select-item-font-size: $font-size-base;
$select-item-bg: $dropdownBackground;
$select-item-fg: $input-color;
$select-option-bg: $dropdownBackground;
$select-option-color: $input-color;
$select-noresults-color: $text-color;
$select-input-bg: $input-bg;
$select-input-border-color: $input-border-color;
$select-menu-box-shadow: $menu-dropdown-shadow;
$select-text-color: $text-color;
$select-input-bg-disabled: $input-bg-disabled;
$select-option-selected-bg: $dropdownLinkBackgroundActive;

@import '../../../node_modules/react-select/scss/default.scss';

@mixin select-control() {
  width: 100%;
  margin-right: $gf-form-margin;
  @include border-radius($input-border-radius-sm);
  background-color: $input-bg;
}

@mixin select-control-focus() {
  border-color: $input-border-focus;
  outline: none;
  $shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075), 0 0 8px $input-box-shadow-focus;
  @include box-shadow($shadow);
}

// react-select tweaks
.gf-form-input--form-dropdown {
  padding: 0;
  border: 0;
  overflow: visible;

  .Select-placeholder {
    color: $input-color-placeholder;
  }

  > .Select-control {
    @include select-control();
    border-color: $dark-3;

    input {
      min-width: 1rem;
    }

    .Select-clear,
    .Select-arrow {
      margin-right: 8px;
    }

    .Select-value {
      display: inline-block;
      padding: $input-padding-y $input-padding-x;
      font-size: $font-size-md;
      line-height: $input-line-height;
      vertical-align: baseline;
      white-space: nowrap;
    }
  }

  &.is-open > .Select-control {
    background: transparent;
    border-color: $dark-3;
  }

  &.is-focused > .Select-control {
    background-color: $input-bg;
    @include select-control-focus();
  }

  &.is-focused:not(.is-open) > .Select-control {
    background-color: $input-bg;
    @include select-control-focus();
  }

  .Select-menu-outer {
    border: 0;
    width: auto;
  }

  .Select-option.is-focused {
    background-color: $dropdownLinkBackgroundHover;
    color: $dropdownLinkColorHover;

    &::before {
      position: absolute;
      left: 0;
      top: 0;
      height: 100%;
      width: 2px;
      display: block;
      content: '';
      background-image: linear-gradient(to bottom, #ffd500 0%, #ff4400 99%, #ff4400 100%);
    }
  }
}

.gf-form-input--form-dropdown-right {
  .Select-menu-outer {
    right: 0;
  }
}