title: Change background color of button when Bootstrap 4 collapse element is open
body: <p>Is it possible to change the background color of a button that opens a collapse card?  My goal is to highlight the button that corresponds to the active data target.</p>
  
  <p>As an example, here is the button that I want to highlight, but only if it's data target (div id january) is opened.  If the data target (div id february) is opened the january button is no longer highlighted and the february button is highlighted. If no data targets are open, no buttons are highlighted.</p>
  
  <pre><code>    &lt;!-- January --&gt;
  
          &lt;div class="card col-sm-2"&gt;
              &lt;button class="btn btn-link btn-block" type="button" data-toggle="collapse" 
               data-target="#january" aria-expanded="true" aria-controls="january"&gt;
                  &lt;i class="fa fa-chevron-circle-right" aria-hidden="true"&gt;&lt;/i&gt;
                    January
              &lt;/button&gt;
          &lt;/div&gt;
  </code></pre>