A CSS file contains all your style definitions. A style definition is a set of instructions telling a browser how to render a paticular tag. For example, the following style definition tells a browser to render the H1 tag in red:
H1 {color : red};
From the View menu, select the "Resource Manager" sub-menu.
In the Resource Manager sub-menu, select "Cascading Style Sheets".
Near the top left of the Cascading Style Sheets Resource Manager,
click .
Enter the name of the new CSS file.
Click "OK".