wpgraphql-global-styles.php

<?php

/**
 * Provides the css generated by theme.json in WPGraphQL
 */

add_action( 'graphql_register_types', function() {
	$field_config = array(
		'type' => 'String',
		'args' => array(),
		'resolve' => function() {
			return wp_get_global_stylesheet();
		},
	);
	register_graphql_field( 'RootQuery', 'globalStyles', $field_config );
} );

This doesn’t seem to handle the alignment classes, which is expected for WordPress 5.8:

https://make.wordpress.org/core/2021/06/29/on-layout-and-content-width-in-wordpress-5-8/

Curious to see if this is different in 5.9.