Default theme directory
From Online Manual
The default theme directory, /Themes/default, is the directory in which the SMF default theme can be found when SMF is installed. Theme #1 in the Themes List will always point to this directory. You can change or replace the files in this directory, but this might cause problems when modifications or upgrades that expect to find the SMF default theme in the default theme directory. You may set your forum default theme to a different theme from the one in the default theme directory.