CSS describes how HTML elements are to be displayed on screen, paper, or in other media. CSS can control the layout of multiple web pages all at once. External stylesheets are stored in CSS files. This allows you to use existing style sheets to quickly style both HTML preview and HTML outputs.  These will not apply to PDF outputs or ePub outputs.

CSS Screenshot.  Arrow 1 is Name, Arrow 2 is URL, Arrow 3 is set as Default checkbox, arrow 4 is Add or Delete CSS button