You will need some custom css for this which you should add to Theme Options -> Custom code
color: #fff;
opacity: 0.7;
background: none repeat scroll 0% 0% #3A77BE;
width: 28px;
height: 28px;
text-align: center;
display: inline-block;
border-radius: 99%;
line-height: 28px;
margin-top: 7px;
margin-right: 3px;
}
.instagram-link i.fa {
color: #FFF;
text-align: center;
line-height: 28px;
}
Second, in your footer widget you'll need to actually add a link to your instagram profile
<a href="#" class="facebook-link"><i class="fa fa-facebook"></i></a>
<a href="#" class="twitter-link"><i class="fa fa-twitter"></i></a>
<a href="#" class="googleplus-link"><i class="fa fa-google-plus"></i></a>
<a href="#" class="pinterest-link"><i class="fa fa-pinterest"></i></a>
<a href="#" class="rss-link"><i class="fa fa-rss"></i></a>
<a href="#" class="instagram-link"><i class="fa fa-instagram"></i></a>