Default theme - What is it and why can't I delete it
From Online Manual
The "Default" Theme is, actually, not the "default theme." while SMF starts out with it as the theme, the "Default" Theme is only a collection of the default templates. That is, these templates are "fallbacks" in case the current Theme does not have the template.
This is similar to URLs - if you don't specify a domain name, (ie. /style.css) it uses the default (making it www.simplemachines.org/style.css). In this case, it's a bit different, but the concept is the same.
For more details check out Default theme.