public static function setActiveTheme($activeTheme) {
		self::$activeTheme = $activeTheme ?? '';
		$engineInstance = Startup::$templateEngine;
		if ($engineInstance instanceof Twig) {
			$engineInstance->setTheme ( $activeTheme, self::THEMES_FOLDER );
		} else {
			throw new ThemesException ( 'Template engine must be an instance of Twig for themes activation!' );
		}
	}