A theme is a subdirectory to the directory themes. It can contain a set of the templates AShop uses, a set of button images and a color set which overrides the colors selected in the admin panel. The colors are selected in the theme.cfg.php file for each theme which also has settings to determine if the theme should override the templates and button images.
 
The themes directory could contain any number of themes and they will automatically be selectable from the Layout page in Store Configuration.
 
A selected theme currently affects all the pages that are visible to customers and affiliates, but not the wholesale pages.