You are running already at least one WordPress Theme if you have installed wordpress.
… now if you upload any standard unzipped folder containing another theme as standardized and required by WordPress itself to deal with themes plugin in the path :
ftp://your_server.com/www/wp/wp-content/themes/atahualpa/
You then surf to your ACP > Themes and you will be able to switch in a few seconds from your default installation THEME to this atahualpa theme
So if you like this theme that changes its header image on re-visiting by a random case, surf to your ACP and search from therein BytesForAll …before all try direct install – ACP > Menu > Appearance > Themes > Manage Themes – Install Themes … Automaticly, if that doesn’t work … download the Theme plugin that you like to your Pc, unzip, and upload and good luck 🙂