Bootstrap class: .custom-select

<select class="custom-select">
  <option selected>Open this select menu</option>
  <option value="1">One</option>
  <option value="2">Two</option>
  <option value="3">Three</option>
</select>

Preview

Sass source

/* _custom-forms.scss:164 */
.custom-select {
  display: inline-block;
  width: 100%;
  height: $custom-select-height;
  padding: $custom-select-padding-y ($custom-select-padding-x + $custom-select-indicator-padding) $custom-select-padding-y $custom-select-padding-x;
  line-height: $custom-select-line-height;
  color: $custom-select-color;
  vertical-align: middle;
  background: $custom-select-bg $custom-select-indicator no-repeat right $custom-select-padding-x center;
  background-size: $custom-select-bg-size;
  border: $custom-select-border-width solid $custom-select-border-color;
  @if $enable-rounded {
    border-radius: $custom-select-border-radius;
  } @else {
    border-radius: 0;
  }
  @include box-shadow($custom-select-box-shadow);
  appearance: none;

  &:focus {
    border-color: $custom-select-focus-border-color;
    outline: 0;
    @if $enable-shadows {
      box-shadow: $custom-select-box-shadow, $custom-select-focus-box-shadow;
    } @else {
      box-shadow: $custom-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: $custom-select-padding-x;
    background-image: none;
  }

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

  // Hides the default caret in IE11
  &::-ms-expand {
    opacity: 0;
  }
}

/* _custom-forms.scss:431 */
.custom-select {
  @include transition($custom-forms-transition);
}

/* _forms.scss:305 */
.custom-select {
  width: auto;
}

/* _forms.scss:54 */
.custom-select {
  .was-validated &:#{$state},
  &.is-#{$state} {
    border-color: $color;

    &:focus {
      border-color: $color;
      box-shadow: 0 0 0 $input-focus-width rgba($color, .25);
    }

    ~ .#{$state}-feedback,
    ~ .#{$state}-tooltip {
      display: block;
    }
  }
}

Uses variables

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/* _custom-forms.scss:164 */
$custom-select-height
$custom-select-padding-y
$custom-select-padding-x
$custom-select-indicator-padding
$custom-select-padding-y
$custom-select-padding-x
$custom-select-line-height
$custom-select-color
$custom-select-bg
$custom-select-indicator
$custom-select-padding-x
$custom-select-bg-size
$custom-select-border-width
$custom-select-border-color
$enable-rounded
$custom-select-border-radius
$custom-select-box-shadow
$custom-select-focus-border-color
$enable-shadows
$custom-select-box-shadow
$custom-select-focus-box-shadow
$custom-select-focus-box-shadow
$input-color
$input-bg
$custom-select-padding-x
$custom-select-disabled-color
$custom-select-disabled-bg

/* _custom-forms.scss:431 */
$custom-forms-transition

/* _forms.scss:305 */


/* _forms.scss:54 */
$state}
$state}
$color
$color
$input-focus-width
$color
$state}-feedback
$state}-tooltip

More in Bootstrap Custom Forms