Simple put powerfull: adds the ability to reference a css file with a custom field for each article.
Look at the generated HTML source code of your article. Search for
id='extraCss-css'. If you can not find it check your
header.php of your theme and look for
<php wp_head(); ?>. This function has to be present because the plugin add the css this way.
id='extraCss-css' is present in your source code please check the path in the css reference.
It seems that your theme is missing
<body <?php body_class(); ?>> in your
header.php instead of