Form Tools is organized into the following structural components: the Form Tools Core code (the "Core"), Modules, Themes, the API and separate language files. The default theme is included as part of the Core. It is always up to date. The default theme contains the pages for the visual presentation and generation of all Form Tools content.
If you do delete the theme (and let's face, somebody will) the other themes won't function properly and Form Tools will come to a grinding halt. But don't panic, all is not lost: re-download Form Tools and upload the /themes/default folder to your server at the appropriate spot in your Form Tools installation.
As mentioned, the default theme contains all pages needed to display Form Tools pages and content. This makes it unique: all other themes don't need to do this. Other themes work by overriding files in the default theme. In other words, as a theme developer all you need to do is override those files that you need. This keeps your themes as simple as can be.