What Is the Default Style Sheet?
The default style sheet is the style sheet used by all newly created notebooks. (New notebooks are called Untitled-n, where n is a number.) The default style sheet is designed to be a general-purpose style sheet for doing interactive calculations and writing simple reports.
If you find that you are editing many cells to change the look of the notebook, you may want to create a different style sheet incorporating all your preferences and make it the default style sheet.