How to use aria-expanded=”true” to change a css property

Why javascript when you can use just css?

a[aria-expanded="true"]{
  background-color: #42DCA3;
}

<li class="active">
   <a href="#3a" class="btn btn-default btn-lg" data-toggle="tab" aria-expanded="true"> 
       <span class="network-name">Google+</span>
   </a>
</li>
<li class="active">
   <a href="#3a" class="btn btn-default btn-lg" data-toggle="tab" aria-expanded="false"> 
       <span class="network-name">Google+</span>
   </a>
</li>