Add Category Name to body_class

functions.php

/* Add Category Name to body_class */
add_filter('body_class','moss_add_category_to_single');

function moss_add_category_to_single($classes) {
if (is_single() ) {
	global $post;
	foreach((get_the_category($post->ID)) as $category) {
		$classes[] = 'category-' . $category->category_nicename;
	}
}
return $classes;
}