title: Conditionally apply bootstrap css class
body: <p>i am learning bootstrap, need help. 
  I have row, then i have a custom_class  which i just need to use when display not extra small (xs). How to do that?</p>
  
  <pre><code>&lt;div class="col-md-7 custom_class"&gt;Left&lt;/div&gt;&lt;div class="col-md-5"&gt;Right&lt;/div&gt;
  </code></pre>
  
  <p>Thanks</p>