Thunder is a Drupal 8 distribution specialised for media houses. It is initially developed by Burda, a German media company, who is now seeking to get other members on the project.
Thunder is not a CMS on it's own right, but a pre-packaged system based Drupal 8 (which uses some Symfony Components). It adds a better default theme, an integration to a video player and a puzzle content provider in a single package.
It's very much a package that is intended to use as-is, similar to the Open Atrium distribution of Drupal 7. This can be very limiting in terms of customisation in the future. Especially the more ready made features you choose to use.