Default theme - What is it and why can't I delete it

From Online Manual

Jump to: navigation, search

The "Default" theme is actually not the "default theme." While SMF starts out with it as the base theme, the "Default" theme is only a collection of the default templates. These templates are "fallbacks" in case the current theme does not have the correct template.

It is similar to URLs - if the base domain name isn't specified, (in example, just /style.css) it uses the default (making the URL, In this case, it works a bit different, but the concept is similar.