twbs--bootstrap/scss/forms/_form-select.scss

// Select
//
// Replaces the browser default select with a custom one, mostly pulled from
// https://primer.github.io/.

.form-select {
  display: inline-block;
  width: 100%;
  height: $form-select-height;
  padding: $form-select-padding-y ($form-select-padding-x + $form-select-indicator-padding) $form-select-padding-y $form-select-padding-x;
  font-family: $form-select-font-family;
  @include font-size($form-select-font-size);
  font-weight: $form-select-font-weight;
  line-height: $form-select-line-height;
  color: $form-select-color;
  vertical-align: middle;
  background: $form-select-bg escape-svg($form-select-indicator) $form-select-background;
  border: $form-select-border-width solid $form-select-border-color;
  @include border-radius($form-select-border-radius, 0);
  @include box-shadow($form-select-box-shadow);
  appearance: none;

  &:focus {
    border-color: $form-select-focus-border-color;
    outline: 0;
    @if $enable-shadows {
      box-shadow: $form-select-box-shadow, $form-select-focus-box-shadow;
    } @else {
      box-shadow: $form-select-focus-box-shadow;
    }

    &::-ms-value {
      // For visual consistency with other platforms/browsers,
      // suppress the default white text on blue background highlight given to
      // the selected option text when the (still closed) <select> receives focus
      // in IE and (under certain conditions) Edge.
      // See https://github.com/twbs/bootstrap/issues/19398.
      color: $input-color;
      background-color: $input-bg;
    }
  }

  &[multiple],
  &[size]:not([size="1"]) {
    height: auto;
    padding-right: $form-select-padding-x;
    background-image: none;
  }

  &:disabled {
    color: $form-select-disabled-color;
    background-color: $form-select-disabled-bg;
    border-color: $form-select-disabled-border-color;
  }

  // Hides the default caret in IE11
  &::-ms-expand {
    display: none;
  }
}

.form-select-sm {
  height: $form-select-height-sm;
  padding-top: $form-select-padding-y-sm;
  padding-bottom: $form-select-padding-y-sm;
  padding-left: $form-select-padding-x-sm;
  @include font-size($form-select-font-size-sm);
}

.form-select-lg {
  height: $form-select-height-lg;
  padding-top: $form-select-padding-y-lg;
  padding-bottom: $form-select-padding-y-lg;
  padding-left: $form-select-padding-x-lg;
  @include font-size($form-select-font-size-lg);
}