Hi Katherine,
If you go to Administration > Site Settings, there is a dropdown list of skins allowing to to select the skin to use. Right next to the dropdown is a link for browsing the skins labelled Preview/Browse, it will open up a window where you can scroll through the designs and see a preview of the site home page using the design.
The files for the skins are located under /Data/Sites/[SiteID]/skins where SiteID is typically 1 for the first site in an installation so it would be under /Data/Sites/1/skins.
Beneath that skins folder are more folders, each folder corresponds to a skin and the folder name is the skin name.
Hope that helps,
Joe