title: Vuetify v-app can't apply class attribute
body: <p>I can't apply <code>class="grey lighten-4"</code> to v-app tag.
  Even color attribute is not working</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;template&gt;
    &lt;v-app class="grey lighten-4"&gt; 
      &lt;Navbar /&gt;
      &lt;v-content class="mx-4"&gt;
        &lt;router-view&gt;&lt;/router-view&gt;
      &lt;/v-content&gt;
    &lt;/v-app&gt;
  &lt;/template&gt;
  </code></pre>
  
  <p>I want to make the content have the "grey lighten-4" color, see screenshot:
  <img src="https://i.stack.imgur.com/hGyqb.png" alt="I want to make the content have the &quot;grey lighten-4&quot; color"></p>