WP Christmas Class allows you to dynamically add a custom CSS class to your body tag during a date range of your choosing.
Please check to make sure the 'body_class()' function is still on your theme's body tag (this is normally found in header.php). If it does not exist, follow the example on the WordPress Codex page when searching for the 'body_class' function.
When adding your custom class styles to the stylesheet, bare in mind that these are over-riding styles and therefore should be added to the end of the stylesheet. You must also take into account stylesheet specificity hierarchy.
To remove the body class, simply disable the plugin or set the date range in the past without the repeat checkbox ticked.