HTML

bootstrap 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>

bootstrap selec

<div class="form-group">
    <label for="exampleSelect">Example select</label>
    <select class="form-control" id="exampleSelect">
      <option value="" selected>1</option>
      <option value="">2</option>
      <option value="">3</option>
      <option value="">4</option>
      <option value="">5</option>
    </select>
</div>

select bootstrap

$(function () {
    $('select').selectpicker();
});

select bootstrap

// To style only selects with the my-select class
$('.my-select').selectpicker();

select bootstrap

// To style all selects
$('select').selectpicker();