How to Create Your Own Style Sheet
You can create your own style sheets instead of using the style sheets included with Mathematica. To reduce the amount of work necessary in creating a style sheet, it is usually best to take a current style sheet that is similar to what you want, copy it, and then edit the copy.
For example, if you want a style sheet like Default.nb, but you want to use a different font for the text cells:
1. Make a copy of Default.nb (located in the Mathematica directory, in SystemFiles/FrontEnd/StyleSheets), called MyDefault.nb.
2. Open MyDefault.nb and go to the cell that defines the style for Text. It is labeled "Prototype for style: Text". Click the cell bracket for that cell.
3. From the Format menu, use the Font submenu and choose the font that will be the default font for Text cells.
4. Save the style sheet.
Now the style sheet MyDefault.nb uses a different font for text cells.
If you want to create a style sheet and simultaneously see how changes to the style sheet will affect a particular notebook, you should first import a private style sheet into the notebook (see Shared and Private Style Sheets). After you are finished editing the style sheet, make the style sheet the active window and then use File Save As to save the style sheet to a separate file. You can then reset the style sheet of the notebook to a shared style sheet.
You can set a personal style sheet to be the default style sheet. (See Selecting the Default Style Sheet.)