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 forum default theme. If you change the files in the default theme directory, then you will have something slightly different from the SMF default theme in that directory. This could cause problems when you wish to install modifications or patches. It may be better to leave the SMF default theme alone, create a copy of it, and make customizations to the copy. For more information about working with themes, see Creating and Understanding Themes.