The most stable theme for Drupal 8 at this time is Bartik. Site works fine with Bartik, although it is possible to improve its appearance and optimize it deleting unnecessary files and scripts. But it is not a good idea to edit Bartik and other core files, this is why you need to copy it and create a new theme.
Clone of Bartik for Drupal 8 is Thisis, download here: https://www.drupal.org/sandbox/tlito/2611000
How to create own theme by copying Bartik: