Cannot install any theme
This is an nginx configuration issue and discussed intensely on this page Yet curiously no solid solution could be offered. Some guy on an other page proposed a solution. I applied that proposed solution and can confirm that in works! 

To apply proposed solution go to your nginx configuration file under 

/etc/nginx/sites-available/ directory on Ubuntu and add

rewrite ^/core/authorize.php/core/authorize.php(.*)$ /core/authorize.php$1;


just above the 

location ~ \..*/.*\.php$ {
    return 403;
}


block. Location does matter!.. Restart your server:


  sudo service nginx restart


and that's it. The problem solved!