Shared and Private Style Sheets
The style sheets listed in the Style Sheet submenu are shared style sheets. A shared style sheet is a style sheet stored in a central location and used by many notebooks at the same time without storing the style definitions within the notebooks. If the style sheet is edited, all notebooks that refer to it are affected. Therefore, care must be taken when editing shared style sheets, especially the ones included with Mathematica.
You can also use private style sheets, which are stored in the notebook and not used by any other notebook. To do this, you must first import a style sheet:
1. In the menu, choose Format Edit Style Sheet.
2. Click Import Private Copy.
This stores a copy of the style sheet within the current notebook.
Any changes made to the imported style sheet are stored in the notebook; they do not affect the centrally stored style sheet. (See Editing a Style Sheet.)