Edit in JSFiddle

$(document).ready(function() {
  $("button#color-toggle").click(function() {
    $("#content p").toggleClass("yellow");
  });
});
<h2>jCombat Demo - jQuery CSS Manipulations</h2>

<div id="content">
  <p>This is a paragraph. <a href="#">Sample Link</a></p>
</div>

<button id="color-toggle">Click here</button>
.yellow {
  background-color: yellow;
}