title: mat-expansion-panel-header background-color on collapse
body: <p>i have some mat-expansion-panels. The header should have a specified color. Ok so on.. but when i expand the Panel, the color changes back to the root color...</p>
  
  <p><a href="https://i.stack.imgur.com/9a76T.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9a76T.png" alt="enter image description here"></a></p>
  
  <p>How can i change this?</p>
  
  <pre><code>&lt;mat-expansion-panel&gt;
    &lt;mat-expansion-panel-header [expandedHeight]="'50px'"  [collapsedHeight]="'50px'" id="test1"&gt;
  
      &lt;mat-panel-title&gt;
          &lt;div class="matPanelSceneSymbol"&gt;&lt;i class="material-icons md-36 md-light active"&gt;weekend&lt;/i&gt;&lt;/div&gt;
  
          &lt;div class="matPanelSceneText"&gt; Testszene&lt;/div&gt;
      &lt;/mat-panel-title&gt;
      &lt;mat-panel-description&gt;
        &lt;div class="matPanelSceneText"&gt;Szenenbeschreibung&lt;/div&gt;
      &lt;/mat-panel-description&gt;
  
    &lt;/mat-expansion-panel-header&gt;
  </code></pre>
  
  <p>.....
  ....</p>
  
  <p>css</p>
  
  
  
  <pre class="lang-css prettyprint-override"><code>/deep/ .mat-expansion-panel-header{
      padding-left: 0px !important;
      padding-top: 1px !important;
  //  background-color: #0070c040  !important;
      padding-bottom: 1px !important;
  }
  
  /deep/ .mat-expansion-panel-header .mat-expanded{
      background-color: none;
  }
  
  #test1{
      background-color:  #0070c040  !important;
  }
  
  #test2{
      background-color:  #8e0fbc40  !important;
  }
  </code></pre>