<div class="b-dropdown-center">
  <!-- ------------------------ -->
  <!-- BOOTSTRAP DROPDOWN START -->
  <!-- ------------------------ -->
  <div class="dropdown">
    <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
      Dropdown button
    </button>
    <div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
      <a class="dropdown-item" href="#">Action</a>
      <a class="dropdown-item" href="#">Another action</a>
      <a class="dropdown-item" href="#">Something else here</a>
    </div>
  </div>
  <!-- ---------------------- -->
  <!-- BOOTSTRAP DROPDOWN END -->
  <!-- ---------------------- -->
  
  
  <!-- ---------------------- -->
  <!--  CUSTOM DROPDOWN START -->
  <!-- ---------------------- -->
  <div class="b-dropdown">
    <select>
      <option>Dropdown button</option>
      <option>Action</option>
      <option>Another action</option>
      <option>Someting else</option>
    </select>
  </div>
  <!-- -------------------- -->
  <!--  CUSTOM DROPDOWN END -->
  <!-- -------------------- -->
</div>
.b-dropdown {
  position: relative;
}

.b-dropdown select {
  color: #fff;
  background-color: #6c757d;
  border-color: #6c757d;
  border-radius: 4px;
  padding: .375rem 1.6rem .375rem .75rem;
  appearance: none;
  outline: 0;
  transition: box-shadow ease 0.3s;
} 

.b-dropdown select:focus {
    box-shadow: 0 0 0 0.2rem rgba(108,117,125,.5);
}

.b-dropdown::after {
    position: absolute;
    top: 50%;
    right: 10px;
    margin-top: -.15em;
    box-sizing: border-box;
    display: inline-block;
    width: 0;
    height: 0;
    margin-left: .255em;
    vertical-align: .255em;
    color: #fff;
    content: "";
    border-top: .3em solid;
    border-right: .3em solid transparent;
    border-bottom: 0;
    border-left: .3em solid transparent;
}





/* --------------------------------------
          Centering the button
----------------------------------------- */

html,
body {
  background-color: #e8fffe;
  width: 100%;
  height: 100%;
}

.b-dropdown-center {
  display: flex;
  align-items: center;
  justify-content: space-around;
  width: 100%;
  height: 100%;
}
Run Pen

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css

External JavaScript

  1. https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.0/jquery.min.js
  2. https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js
  3. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.3.1/js/bootstrap.min.js