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// category id in body and post class
function category_id_class($classes) {
	global $post;
	foreach((get_the_category($post->ID)) as $category)
		$classes [] = 'cat-' . $category->cat_ID . '-id';
		return $classes;
}
add_filter('post_class', 'category_id_class');
add_filter('body_class', 'category_id_class');