Right now there are 13 Drupal8 themes available at drupal.org/project/themes
I put some contributed themes in /apache/drupal8/themes where/apache/drupal8 is the drupal8 root path.
When i put any of those themes into the themes folder described above some of them show up at myinstall.domain/#overlay=admin/appearance (fontfolio, garland, zurb, seven and newsflash, the others not) under the deactivated tab. When i try to activate one of these themes by clicking the "activate and set as default" link it gives me an "theme themename was not found".
When i try to set the "Seven 8.0-dev (admin theme)" theme as active it gives me an error in the page title with a blank overlay.
Putting the themes into /apache/drupal8/sites/multisite.domain/themes it is working fine and so does it work when putting them into the "old" structure /apache/drupal8/sites/all/themes.
The /apache/drupal8/themes/README.txt says that all contrib modules should be placed in this directory but that obviously doesn´t work.