.select option {

 background-color: #FFFFFF;

 padding: 2px;

}

.suboption {

 background-color: #EEEEEE;

 font-size:0.8em;

 padding:10px;

}
</style>
<select class="select">
<option> 1
<option class="suboption"> 1:1
<option class="suboption"> 1:2
<option class="suboption"> 1:3
<option> 2
</select>