add_filter('body_class','add_category_to_single');
function add_category_to_single($classes, $class) {
	if (is_single() ) {
		global $post;
		foreach((get_the_category($post->ID)) as $category) {
			echo $category->cat_name . ' ';
			// add category slug to the $classes array
			$classes[] = 'category-'.$category->slug;
		}
	}
	// return the $classes array
	return $classes;
}