title: Change color of Angular material expansion header on hover
body: <p>I have an Angular Material 7 Expansion Panel. When I hover over one of the <strong>mat-expansion-panel-header</strong> the color is changing. How can I change this color?</p>
  <p>Code:</p>
  <pre><code>&lt;mat-accordion&gt;
      &lt;mat-expansion-panel&gt;
            &lt;mat-expansion-panel-header&gt;
              &lt;mat-panel-title&gt;
                Titel
              &lt;/mat-panel-title&gt;
              &lt;mat-panel-description&gt;Hi&lt;/mat-panel-description&gt;
            &lt;/mat-expansion-panel-header&gt;
            Content
      &lt;/mat-expansion-panel&gt;
  &lt;/mat-accordion&gt;
  </code></pre>