User.php

<?php

namespace RunThroughHistory\WPGraphQL\Type\Object;

use WPGraphQL\Model\User as UserModel;
use RunThroughHistory\Interfaces\Hookable;

class User implements Hookable {

    public function register_hooks() {

        add_action( 'graphql_register_types', [ $this, 'register_fields' ] );

    }

    public function register_fields() {

        register_graphql_fields( 'User', [
            
            'prefersColorScheme' => [
                'type'        => 'String',
                'description' => __( 'User\'s preferred color theme', 'run-through-history' ),
				'resolve'     => function( UserModel $user ) {
                    return wp_get_user_meta( $user->fields['userId'], 'prefers_color_scheme' );
				},
            ],

        ] );
        
    }
}

UserUpdate.php

<?php
namespace RunThroughHistory\WPGraphQL\Mutation;

use GraphQL\Error\UserError;
use GraphQL\Type\Definition\ResolveInfo;
use WPGraphQL\AppContext;
use RunThroughHistory\Interfaces\Hookable;

class UserUpdate implements Hookable {

    public function __construct() {
        
    }

    public function register_hooks() {
        add_action( 'graphql_register_types',                              [ $this, 'register_input_fields'] );
        add_action( 'graphql_before_resolve_field',                        [ $this, 'validate_fields' ], 10, 7 );
        add_action( 'graphql_user_object_mutation_update_additional_data', [ $this, 'save_additional_data' ], 10, 3 );
    }

    public function register_input_fields() {
        register_graphql_fields( 'UpdateUserInput', [
            'prefersColorScheme' => [
                'type'        => [ 'non_null' => 'String' ],
                'description' => __( 'User\'s preferred color theme', 'run-through-history' ),
            ],
        ] );
    }

    /**
     * Fire an action BEFORE the field resolves
     *
     * @param mixed           $source         Source passed down the Resolve Tree.
     * @param array           $args           Args for the field.
     * @param AppContext      $context        AppContext passed down the ResolveTree.
     * @param ResolveInfo     $info           ResolveInfo passed down the ResolveTree.
     * @param mixed           $field_resolver Field resolver.
     * @param string          $type_name      Name of the type the fields belong to.
     * @param string          $field_key      Name of the field.
     * @param FieldDefinition $field          Field Definition for the resolving field.
     */
    public function validate_fields( $source, array $args, AppContext $context, ResolveInfo $info, $field_resolver, string $type_name, string $field_key ) : void {
        if ( 'RootMutation' !== $type_name || ! $this->is_update_user_mutation( $field_key ) ) {
            return;
        }

        if ( ! $this->is_preferred_color_scheme_valid( $args ) ) {
            throw new UserError( 'An invalid color scheme name was provided. Accepted values are \'light\' or \'dark\'.' );
        }
    }

    private function is_preferred_color_scheme_valid( array $args ) : bool {
        return in_array( $args['input']['prefersColorScheme'], [ 'light', 'dark' ] );
    }

    /**
     * @param int    $user_id       The ID of the user being mutated.
     * @param array  $input         The input for the mutation.
     * @param string $mutation_name The name of the mutation (ex: create, update, delete).
     */
    public function save_additional_data( int $user_id, array $input, string $mutation_name ) : void {
        
        if ( ! $this->is_update_user_mutation( $mutation_name ) ) {
            return;
        }

        $prefers_color_scheme_sanitized = sanitize_text_field( $input['prefersColorScheme'] );

        update_user_meta( $user_id, 'prefers_color_scheme', $prefers_color_scheme_sanitized );

    }

    private function is_update_user_mutation( string $mutation_name ) : bool {
        return 'updateUser' === $mutation_name;
    }
}