This allows you to register your own themes in the customizer and select them in the customizer.

/**
 * Here you can add your own themes to Helpful.
 *
 * @param array $themes default themes.
 * @return array
 */
function custom_function( $themes ) {
	$themes[] = [
		'id'         => 'custom',
		'label'      => __( 'Custom', 'textdomain' ),
		'stylesheet' => 'STYLESHEET_URL',
	];

	return $themes;
}

add_filter( 'helpful_themes', 'custom_function' );