bootstrap select dropdown css example
Example: selectpicker dropdown in bootsttrap
<select class="selectpicker" required multiple>
<optgroup label="Picnic">
<option>Mustard</option>
<option>Ketchup</option>
<option>Relish</option>
</optgroup>
<optgroup label="Camping">
<option>Tent</option>
<option>Flashlight</option>
<option>Toilet Paper</option>
</optgroup>
</select>