Go to Admin > Manage Styles Click the Create tab. Type a name for the new style. Scroll to the bottom. Click the save button.

Click the Modify Style tab. Choose the style you created Click Pick Style

Scroll to the bottom Erase the existing css Paste the new code in.  Click Modify Style

To use your new style: Choose it in the dropdown menu from Manage Styles.

If you have problems with this use the Stylesheet Request and Support Board.

Related Links:

Stylesheet Index
Holiday Stylesheets       

Thinking of creating a stylesheet yourself?  Lots of tips and helpful links here:
Stylesheet Making Tips/Helpful Links
Admin CP Manage Styles Guide

If you want to request a stylesheet or have questions about making stylesheets be sure to use this topic at Support:
Stylesheet Requests and Support