title: My flexbox won't center
body: <p>I'm currently trying to learn how to make websites, I just started to test out flexbox but I cant figure out how to center the red box:</p>
  
  <p><img src="https://i.stack.imgur.com/YF0v2.png" alt="(See picture)"> </p>
  
  <p>I looked at a very clear guide (<a href="https://css-tricks.com/snippets/css/a-guide-to-flexbox/" rel="nofollow noreferrer">https://css-tricks.com/snippets/css/a-guide-to-flexbox/</a>) and I figured I just had to use justify-content: center. But I am cleary doing something wrong please help.</p>
  
  <p>P.S. Run the snippet in fullscreen, otherwise you won't see the problem.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>* {
    margin: 0px;
    padding: 0px;
  }
  .flex_container {
    display: flex;
    flex-flow: row wrap;
    border: 5px solid blue;
  }
  header,
  section,
  footer,
  aside,
  nav,
  article {
    display: block;
  }
  body {
    justify-content: center;
    border: 5px solid grey;
  }
  #top_menu {
    text-align: center;
    width: 100%;
    height: 40px;
    padding: 10px;
    border: 5px solid green;
  }
  #top_menu li {
    list-style: none;
    font: 20px helvetica;
    color: green;
    display: inline;
  }
  main {
    padding: 20px;
    max-width: 1000px;
    border: 5px solid red;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="flex_container"&gt;
    &lt;nav id="top_menu"&gt;
      &lt;ul&gt;
        &lt;li&gt;Tab 1&lt;/li&gt;
        &lt;li&gt;Tab 2&lt;/li&gt;
        &lt;li&gt;Tab 3&lt;/li&gt;
      &lt;/ul&gt;
    &lt;/nav&gt;
  &lt;/div&gt;
  &lt;main&gt;
    Sample text
  &lt;/main&gt;</code></pre>
  </div>
  </div>
  </p>