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 SMF default theme, none of the other themes would work. In fact, you should probably avoid making any changes at all in the default theme directory -- that might 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 a different forum default theme. You can even stop them from seeing the default theme at all, using Themes and Layout >> Manage and Install.
If a mess in a custom theme prevents you from using the forum, you will always have the SMF default theme to fall back on. Just use ?theme=1 in your URL. 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