title: W3.CSS how to make container centered inside another container
body: <p>I'm using the w3.css, I wanted to make a colored background with width 100%,<br>
  and inside it another box with width 80% which will contain the text, so I used the following:</p>
  
  <pre><code>&lt;div class="w3-container w3-light-green"&gt;
  &lt;div class="w3-container w3-sand w3-center" style="width:80%"&gt;
  test
  &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>The problem is that the second container with the 80% width is not centered in the page although I used w3-center.
  I noticed that w3-center only centers the text, but the container itself isn't centered, so how can I center the container block?</p>