custom-preferences-page.php

<?php

// Load after Groundhogg
use function Groundhogg\is_managed_page;

add_filter( 'template_include', 'load_custom_preferences_page', 11 );

/**
 * Load the custom template from the them folder
 * 
 * @param $template
 * 
 * @return string
 */
function load_custom_preferences_page( $template ){

	if ( ! is_managed_page() ) {
		return $template;
	}

	$page = get_query_var( 'subpage' );

	if ( $page !== 'preferences' ) {
		return $template;
	}
	
	// Path to your custom preferences page...
	return get_theme_file_path( 'preferences.php' );
	
}