How to change the Bootstrap accordion header on collapse/collapsed

When all panels are collapsed, the background color should be white. When the panel is open, the panel title background color should be grey. How can I do this, please?

      <div class="panel panel-simple">
        <div class="panel-heading" role="tab" id="headingOneMaintenance">
          <h4 class="panel-title">
            <a class="accordion-toggle" role="button" data-toggle="collapse" data-parent="#accordionMaintenance" href="#collapseOneMaintenance" aria-expanded="true" aria-controls="collapseOneMaintenance">
              What will you do to prepare my apartment before I move in? 
            </a>
          </h4>
        </div>
        <div id="collapseOneMaintenance" class="panel-collapse collapse in" role="tabpanel" aria-labelledby="headingOneMaintenance">
          <div class="panel-body">
            Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
          </div>
        </div>
      </div>

With this CSS, all my panels are white, but when the panel is open, I want the header to be gray:

.panel-heading {
   background-color: white;}

It doesn't have to be this CSS though. Thanks!

You can do a little hack to .accordion-toggle with CSS margins & paddings to cover all panel dimensions:

Snippet below:

.accordion-toggle {
  background-color: gray;
  display: block;
  padding: 10px;
  margin: -10px -15px;
  border-top-left-radius: 4px;
  border-top-right-radius: 4px;
}

.accordion-toggle.collapsed {
  background-color: white;
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<div class="panel panel-simple">
  <div class="panel-heading" role="tab" id="headingOneMaintenance">
    <h4 class="panel-title">
      <a class="accordion-toggle" role="button" data-toggle="collapse" data-parent="#accordionMaintenance" href="#collapseOneMaintenance" aria-expanded="true" aria-controls="collapseOneMaintenance">
        What will you do to prepare my apartment before I move in?
      </a>
    </h4>
  </div>
  <div id="collapseOneMaintenance" class="panel-collapse collapse in" role="tabpanel" aria-labelledby="headingOneMaintenance">
    <div class="panel-body">
      Anim pariatur cliche reprehenderit, enim eiusmod high life accusamus terry richardson ad squid. 3 wolf moon officia aute, non cupidatat skateboard dolor brunch. Food truck quinoa nesciunt laborum eiusmod. Brunch 3 wolf moon tempor, sunt aliqua put a bird
      on it squid single-origin coffee nulla assumenda shoreditch et. Nihil anim keffiyeh helvetica, craft beer labore wes anderson cred nesciunt sapiente ea proident. Ad vegan excepteur butcher vice lomo. Leggings occaecat craft beer farm-to-table, raw
      denim aesthetic synth nesciunt you probably haven't heard of them accusamus labore sustainable VHS.
    </div>
  </div>
</div>