The Stylesheets palette creates notebooks of a particular style or applies stylesheets to open notebooks.
To create a notebook of a particular style, select a stylesheet category from the drop-down menu at the top of the palette. Then open a new styled notebook by moving your mouse over a thumbnail and clicking New.
If you want to pick a stylesheet for an existing notebook, select a stylesheet category from the drop-down menu at the top of the palette. Then apply the stylesheet to an open notebook by moving your mouse over a thumbnail and clicking Apply.