Default theme - What is it and why can't I delete it
From Online Manual
The "Default" theme is not, actually, the "default" theme. Whilst SMF starts out with it as the base theme, the "Default" theme is only a collection of the default templates. These templates are "fallbacks". If a custom theme doesn't contain some required files, the software will use the corresponding files from the default theme's directory.
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, www.simplemachines.org/style.css). In this case, it works a bit different, but the concept is similar.