Today I found one question in MS CRM Development Forum to format Left navigation item, we can easily do this with JS. Just use below code on form load. I have tested this in contact form to form “More Addresses”

var Item=document.getElementById(‘navAddresses’);

if(Item!=null){
Item.style.fontWeight = ‘bold’;
Item.style.color = ‘#ff0000’;}

and result will be  as below