Change icons used for SharePoint 2016 and SharePoint 2016 (spcommon.png and fgimg.jpg)

Designer can easily change icons used for site by modifying the following sprite, one example if you want to have the ribbon in a dark colour the icons "share" "follow" are nearly invisible because they are gray. 

 

Once you've changed the image you can use the following css to override the image where they are used : 

img[src*="fgimg.png"]{ 
background-image:url("/_layouts/15/[YOUR-PROJECT]/fgimg-project.png"); 
width:0; 
height:0; 
padding:346.5px 90px; 
}

img[src*="spcommon.png"]{ 
background-image:url("/_layouts/15/[YOUR-PROJECT]/spcommon-project.png"); 
width:0; 
height:0;
padding:134px 135.5px;
}