core-bundle/src/Resources/contao/classes/Backend.php
public static function getTheme()
	{
		$theme = Config::get('backendTheme');
		$rootDir = System::getContainer()->getParameter('kernel.project_dir');

		if ($theme != '' && $theme != 'flexible' && is_dir($rootDir . '/system/themes/' . $theme))
		{
			return $theme;
		}

		return 'flexible';
	}
public static function getTheme()
	{
		$theme = Config::get('backendTheme');
		$rootDir = System::getContainer()->getParameter('kernel.project_dir');

		if ($theme != '' && $theme != 'flexible' && is_dir($rootDir . '/system/themes/' . $theme))
		{
			return $theme;
		}

		return 'flexible';
	}
Return the current theme as string

@return string The name of the theme