Themes are structured in a very similar manner to the core script. They are all located in their own unique folder, in the /themes folder.
If you look in the Form Tools root folder, you'll see two folders:
which look something like this:
/admin/account/ /admin/clients/ /admin/forms/add/ /admin/forms/edit/ /admin/forms/option_lists/ /admin/modules/ /admin/settings/ /admin/themes/ /clients/account/ /clients/forms/
Now look in the default theme folder (/themes/default).
/default/admin/account/ /default/admin/clients/ /default/admin/forms/add/ /default/admin/forms/edit/ /default/admin/forms/option_lists/ /default/admin/modules/ /default/admin/settings/ /default/admin/themes/ /default/clients/account/ /default/clients/forms/
As you can see, it mimics the directory structure exactly. This is done for all pages that can be seen in a web browser.
So, when you're looking for the Smarty template that renders a particular page, you know exactly where to find it. There is a little leeway provided for theme developers in that you can create your own include files to be referenced wherever you want, but by and large the goal was to make the PHP-Smarty page relationship 1-1 to make locating the Smarty template as simple as possible.