Theme - is a common design layout of the whole web site, a combination of templates.
There is a set of ready Themes provided within the software. So you can select one these Themes to be used for the front-end design according to your liking.
Also the administrator is given with the opportunity to create new Themes using one of the existing Themes as a sample.
You can create an unlimited number of new Themes for your site, but you can use only one at a time for the front-end.
Be default all Themes basically consist of the same templates.
Themes differ in style, color spectrum, sometimes in layout - so that administrator would have options to choose from.