title: Bootstrap 4: Buttongroup - change active color
body: <p>I have a buttongroup with bootstrap 4 and everything is fine</p>
  
  <pre><code>&lt;div class="btn-group" role="group" aria-label="Basic example"&gt;
    &lt;button type="button" class="btn btn-outline-secondary"&gt;Left&lt;/button&gt;
    &lt;button type="button" class="btn btn-outline-secondary"&gt;Middle&lt;/button&gt;
    &lt;button type="button" class="btn btn-outline-secondary"&gt;Right&lt;/button&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>How can I highlight the selected state more? E.g. I want a red background color of the active button.</p>