Editing Shared or Private Style Sheets
First, it is important to decide whether you want to edit the centrally stored style sheet or to import a private copy. If you want to affect the display of many notebooks, you should edit the centrally stored style sheet. If you want to affect the display of only one notebook, you should import a private copy and edit it.
You must consider carefully if you want to edit the shared style sheets included with Mathematica. While you can always retrieve a copy of the style sheet from the CD should you make an error, it is better to make a copy of the style sheet and edit the copy. (See How to Create Your Own Style Sheet.)
If you click Format Edit Style Sheet and the notebook currently uses a shared style sheet, a dialog box appears. You have the following options.
Import Private Copy. By clicking Import Private Copy, a copy of the style sheet currently in use will be used and stored privately with the notebook. Changes to this style sheet do not affect the centrally stored style sheet, and changes made to the style sheet are instantly reflected in the open notebook.
Edit Shared Style Sheet. By clicking Edit Shared Style Sheet, the centrally stored style sheet will be opened. Any changes to the style sheet will be applied to all notebooks using that style sheet, but notebooks that use the style sheet and are open will not be updated automatically.
You can open the style sheet directly using the File Open command; this will cause all changes in the style sheet to be applied to all notebooks using that sheet.