title: I want to change background color when change screen size at Bootstrap 3
body: <p>I'm using bootstrap 3. I have a header with transparent red background, but I want to change it according to screen size changes.</p>
  
  <p>For example:</p>
  
  <ul>
  <li>At lg size, background-color will be red</li>
  <li>At sm size, background-color will be pink</li>
  </ul>