title: Reference a class/mixin without completely importing the LESS file
body: <p>I'm using the roots theme for wordpress: <a href="https://github.com/retlehs/roots/" rel="noreferrer">https://github.com/retlehs/roots/</a></p>
  
  <p>It already comes with a precompiled bootstrap.css and recommends to use app.css for any customizations. As I don't have the options to add the classes to the buttons via html or javascript, I'd like to use the LESS sources to reference a css class, for example, I want to give a submit button the bootstrap button style:</p>
  
  <pre><code>input#submit{
  .btn;
  .btn-primary;
  }
  </code></pre>
  
  <p>If I use <code>@import 'bootstrap.less';</code> it adds the entire bootstrap css into app.css. How can I just use the bootstrap.less as reference for compiling?</p>