title: How to change a css class property value for a specific mediaquery?
body: <p>I have the class </p>
  
  <pre><code>.foo {
    width: 30px;
  }
  @media only screen and (max-width:768px){
     .foo {
       width: 20px;
     }
  }
  </code></pre>
  
  <p>And I want to change the value of width from 20px to 15px with javascript for the mediaquery only.
  When I do it, it changes the class for all resolution discounting the mediaquery in the CSS.</p>