The SMF default theme is the default look-and-feel theme supplied with the SMF installation in the default theme directory. It may be different from the "overall forum default theme". If you change the files in the default theme directory, you may make upgrade and mod installation difficult -- it may be better to create a second theme as a copy of the default, set it as the overall forum default, and make changes to the second theme. For more information about working with themes, see Creating and Understanding Themes.