functions.php

// Add custom post class to posts in the "Featured" category
add_filter('post_class', 'eo_custom_post_class');
function eo_custom_post_class($classes)
{
  $new_class = 'featured-post';
  if (in_category('Featured'))
    $classes[] = esc_attr(sanitize_html_class($new_class));

  return $classes;
}