How to Add Category Id to WordPress Body and Post Class This snippet is very handy if you wish to add category ID to WordPress body and post class for ex. targeting CSS styles and such.

add-body-class-wordpress_functions.php

function category_id_class($classes) {
	global $post;
	foreach((get_the_category($post->ID)) as $category)
    $classes [] = 'cat-' . $category->cat_ID . '-id';
    //if you want to add category slug replace below code
		//$classes [] = 'cat-' . $category->slug;           
	return $classes;
}
add_filter('post_class', 'category_id_class');
add_filter('body_class', 'category_id_class');