How do I apply it? I don't see any instructions, and I've never applied a theme from a .zip file before. Using Ubuntu 13.04
The instructions are on the download page here
Ubuntu, Linux Mint, & Ubuntu derivatives: 13.04 Raring and 13.10 Saucy Add this PPA, PPA may take several hours to sink new package updates. Please be patient. copy and paste the following in a terminal:
sudo add-apt-repository ppa:killhellokitty/themes.ppa
sudo apt-get update
sudo apt-get install delorean-dark-stripped-themes-3.8
This will add a repository and let you install a package with the theme in it, which is probably easier/better, because you will also be able to get updates for the theme as well.
If you really want to use the .zip file, that question is answered in detail here, but essentially you just extract the contents of the theme to a directory .theme in your home directory and use the program, MyUnity to to apply the theme.