To achieve the desired effect, add the fa-circle icon on the stack, which will be transparent on the display ( opacity:0.0 ) and solid ( opacity:1.0 ) on :hover .
For example (also on JSFiddle: http://jsfiddle.net/2hxxuv52/5/ ):
HTML
<span class="fa-stack fa-sm"> <i class="fa fa-circle fa-stack-2x "></i> <i class="fa fa-circle-thin fa-stack-2x"></i> <i class="fa fa-user-plus fa-stack-1x "></i> </span>
CSS
.fa-stack .fa { color: black; } .fa-stack .fa.fa-circle-thin { color: black; } .fa-stack .fa.fa-circle { opacity:0.0; color:black; } .fa-stack:hover .fa.fa-user-plus { color: white; } .fa-stack:hover .fa.fa-circle-thin { color: black; } .fa-stack:hover .fa.fa-circle { opacity:1.0 }
source share