Saying that you don't. LiteStep is the most powerfull and advanced replacement shell you can get. There will more than likely be at least one part of a new theme that you can't support with your installation. Click Download Now and it will get everything it needs to load. Then you should be fine. There have been grumblings in the past on the forums about some MacOS themes from that site that don't work. Please read the comments from the theme, on that site, and see what others have experienced.