title: Change background color of header row for element-ui
body: <p>I am using <code>element-ui</code> for adding a table with some data in a Vue project. I wish to change the background color only for the header row. How can I achieve that?</p>
  <p>Things I've tried:</p>
  <p>Added a custom class to <code>:row-class-name</code> prop:</p>
  <pre class="lang-html prettyprint-override"><code>&lt;el-table :row-class-name=&quot;headerStyle&quot;&gt;
  </code></pre>
  <p>(In methods)</p>
  <pre><code>headerStyle() {
     return 'customClass'
  }
  </code></pre>
  <p>In style tags in the same <code>.vue</code> file:</p>
  <pre><code>.el-table .customClass {
  /*Custom CSS*/
  }
  </code></pre>