The Jednotka WordPress theme was for some time quite a popular theme for WordPress. However, the last update for it was published in December 2014, and there were no more subsequent updates. That basically meant it wasn’t compatible with PHP 7, and it broke down on anything that recent.
Up untill October 2018 it was still for sale on that Envato website. To me that was quite in bad taste. People would buy a theme that wasn’t even working on current PHP versions. Apparently that is how you make money, by cheating people out of it.
Last October I met someone at a local WordPress meetup who was using the Jednotka theme on het website, but had problems with it. A lot of admin pages broke down and the WordPress dashboard was quite useless.
I helped her with a few bugfixes and updates for the this theme that make it work with PHP 7.1 and 7.2. You can find that updated theme over at Codeberg.