How to create theme in magento 2 stable version?

Any a module, theme or language in Magento2 stable, you need must create a registration.php file in directory contain module/theme/language. Autoload of Composer it load all file registration.php before Magento2 run.

A registration.php file of a theme the following in app/design/<area>/<Vendor>/<theme_name>/:

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
  \Magento\Framework\Component\ComponentRegistrar::THEME,
'<area>/<VendorName>/<theme_name>',  
__DIR__
);