Default theme - What is it and why can't I delete it
From Online Manual
The short answer is -- because, even if you have several other custom themes installed, these themes almost certainly still depend on files in the default theme. If you deleted the default theme, none of the other themes would work. In fact, some kinds of changes you might make to these files could cause some custom themes not to work properly, or make it difficult to install modifications.
If you don't want your members and guests to see the default theme, you can set your forum default theme to the theme you'd like to use. You can even control whether your members can use the default theme at all, at [[SMF2.0:Themes_and_Layout#Manage and Install|Themes and Layout >> Manage and Install].
Do you want to know more? Well, there are actually several ways the term "default theme" is used in SMF.
First, there is the SMF default theme that comes with every fresh SMF install. It is the look and feel designed for this version of SMF. Any modifications you might decide to install will be compatible with the SMF default theme, and some of them may make small changes to the SMF default theme files. This is theme #1 in your SMF forum.
Second, there is the default theme directory, or Themes/default. This is where the SMF default theme files are stored. Any custom theme you might install is actually a collection of theme files in a new directory. If a custom theme does not contain some required files, the software will use the corresponding files from the default theme directory. This means the default theme directory is a base or fallback for every other theme. If the files weren't there (or if they were very much changed), the custom theme you installed would not work as it was designed to do.
Third, there is the forum default theme. This is what you, as administrator, set as "overall forum default" at [[SMF2.0:Themes_and_Layout#Manage and Install|Themes and Layout >> Manage and Install]. It is the theme guests will see, and the theme members will see until they choose one of the other themes you have installed (if you give them permission to do this).
If you mess up customizing a theme and can't use the forum, try again with index.php?theme=1 in your URL. Because you preserved the default theme in its original state, you have at least one theme that still works. If you install a mod that "breaks" the default theme, you are going to have to get a working set of files back again. For more on this, please see A_mod_broke_my_forum