To create a new style, go to Admin Panel -> Themes -> Manage Templates and find "Create a new theme called ____ as [stylesheet only]" at the bottom. Give your style whatever name you'd like, and leave the selection as stylesheet only if all you're going to customize is the CSS. After you click "create", your new style will be available on the theme selector at the top of the page. Select it as your theme and then click "Stylesheet" to edit. Since this is a style-only theme, all the other templates will be from your preexisting theme.

Visitors will be able to select your new style at will by editing their profile or using the style selector. To make this new style the default style, go to Admin -> Themes -> Theme Settings and select it from the list for the default style setting.

To delete a style, you can use the delete themes option at the bottom of Admin -> Themes -> Add Themes.