Had the same problem, after downloading the master.zip from github. This problem ist described in the troubleshooting section on github:
Sounds like you don't have our default theme - Casper, your content/themes/casper folder is probably empty. When cloning from Github be sure to use SSH and to run git submodule update --init. Don't use the release zip file on Github, if you want a zip file use the one from Ghost.org.
from https://github.com/TryGhost/Ghost/blob/master/CONTRIBUTING.md
So instead of the Zip from Github, use this one: https://ghost.org/zip/ghost-0.4.2.zip