Style Sheet File Locations
The style sheets listed in the Style Sheet submenu are in the main Mathematica directory, in SystemFiles/FrontEnd/StyleSheets. You can store style sheets that you have created in one of the following locations:
The preferences directory. This is usually the best place to store style sheets on Unix systems. Storing notebooks here prevents them from being deleted when you uninstall Mathematica.
The configuration directory. This is usually the best place to store style sheets on Macintosh or Microsoft Windows systems.
Typical values for the preferences and configuration directories.
The style sheets should be stored in a subdirectory called FrontEnd/StyleSheets. These style sheets are then available from the Style Sheet submenu.
The global option StyleSheetPath gives the list of directories that are searched for style sheets upon startup.
If you uninstall Mathematica, you should first make backup copies of your style sheets, especially if they are stored in the configuration directory.