Rate/VotingAPI - Add class to button if user has voted
This change will only work for "Thumbs up" widget. You can place this code in your template.php under your custom theme or in a custom module.

function MODULE_NAME_OR_TEMPLATE_NAME_process_rate_template_thumbs_up(&amp;$variables) {
  extract($variables);

  $classes = 'rate-thumbs-up-btn-up';

  if (isset($results['user_vote']) &amp;&amp; $results['user_vote'] == $links[0]['value']) {
    $classes .= ' user-voted';
  } else {
    $classes .= ' user-not-voted';
  }

  $variables['up_button'] = theme('rate_button', array('text' =&gt; $links[0]['text'], 'href' =&gt; $links[0]['href'], 'class' =&gt; $classes));
}


Make sure to replace MODULE_NAME_OR_TEMPLATE_NAME with an actual template name or module name.