themes-wordpress/kansei/patterns/404.php

<?php
/**
 * Title: A 404 page
 * Slug: kansei/404
 * Inserter: no
 */

declare( strict_types = 1 );
?>

<!-- wp:heading {"style":{"spacing":{"margin":{"top":"0","right":"0","bottom":"0","left":"0"}},"typography":{"fontStyle":"normal","fontWeight":"700"}},"fontSize":"small"} -->
<h2 class="wp-block-heading has-small-font-size" style="margin-top:0;margin-right:0;margin-bottom:0;margin-left:0;font-style:normal;font-weight:700"><?php echo esc_html__( 'Oops, we lost you...', 'kansei' ); ?></h2>
<!-- /wp:heading -->