body_classに固定ページのルート、投稿タイプのスラッグ、カテゴリースラッグを追加する

functions.php

<?php
function add_page_root_body_class( $classes ) {
		$post_type = get_query_var( 'post_type' );
		if ( is_page() ) {
			global $post;
			if ( $post->ancestors ) {
				$root = $post->ancestors[count($post->ancestors) - 1];
				$root_post = get_post( $root );
				$classes[] = esc_attr( $root_post->post_name );
			} else {
				$classes[] = esc_attr( $post->post_name );
			}
		} elseif ( $post_type ) {
			global $post;
			$classes[] = esc_attr( $post_type );
		} elseif ( is_single() ) {
			$cat       = get_the_category();
			$cat_slug  = $cat[0]->category_nicename;
			$classes[] = esc_attr( $cat_slug );
		} elseif ( is_category() ) {
			$cat_id  = get_query_var('cat');
			$cat     = get_category($cat_id);
			$cat_slug  = $cat->category_nicename;
			$classes[] = esc_attr( $cat_slug );
		}
	return $classes;
}
add_filter( 'body_class', 'add_page_root_body_class' );