Include category id in body_class() and post_class()

Use this code on your themes functions.php to add current category id on body and post

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');