We are unable to assist with custom development and design, so I will move this to our community support area. I know I have done this before by targeting the ID of the navigation item, similar to this (In this instance, 34 is the ID. If you right click then inspect the navigation item, you will be able to get the ID, and the content item there is the icon
.ipsNavBar_primary > ul > li#elNavSecondary_34 > a:before
{
font-family: FontAwesome;
font-size:15px;
font-weight: normal;
margin-right: 5px;
content: "\f041";
}