title: Angular 2+ theme button colors not working
body: <p>I have a stackblitz example <a href="https://stackblitz.com/edit/angular-zdzuv9" rel="noreferrer">here</a>
  that I believe is set up following the <a href="https://material.angular.io/guide/theming" rel="noreferrer">Angular theming documentation</a>, and yet the colors in the following code are not displaying:</p>
  
  <pre><code>&lt;button mat-raised-button color="primary"&gt;Primary&lt;/button&gt;
  &lt;button mat-raised-button color="accent"&gt;Accent&lt;/button&gt;
  &lt;button mat-raised-button color="warn"&gt;Warn&lt;/button&gt;
  &lt;button mat-raised-button disabled&gt;Disabled&lt;/button&gt;
  </code></pre>
  
  <p>What's more, I'm brand new to stackblitz and can't find where the body tag is (it's not in index.html?!), so I assigned the my-app tag the class,"mat-app-background":</p>
  
  <pre><code>&lt;my-app class="mat-app-background"&gt;loading&lt;/my-app&gt;
  </code></pre>
  
  <p>I have looked through several SO entries (e.g. <a href="https://stackoverflow.com/questions/46599971/angular2-material-theming-does-not-work">this</a>)and github issues pages (e.g., <a href="https://github.com/angular/material2/issues/1798" rel="noreferrer">this</a>), and none seem to be relevant to my issue or current. Any help is much appreciated.</p>