@matouschek : You will first need to download the theme from My-Account page. Then Log in to the WordPress Administration Panel.
Go to “Appearance => Themes => Add New”
Click on “Uploads” and Browse the downloaded file Fotografie.zip from the desktop where you have it downloaded
Click on “Install Now” and then “Activate” to install the Theme.
Follow this link to know more about installing and activating the theme. Let me know if you have any more issues.