I did a default installation on our Linux server with the blackcandy theme. After creating some content but not publishing any of it, I changed to the higherground theme, did a ?flush=1 and I can see the new look. However, now when I go to admin and the url changes to security/login there is no login page. I am presented with the same page as the home page. If I change back to blackcandy, I can login.
Looks like I somehow deleted the piece of code that displays the main content.