Setting theme (or stylesheets) based on URL

Is it possible to make a theme or certain css files load depending on the URL that a user goes to?

For example, if a user goes to http://www.samplesite.com/design1, design1.css loads. If the user goes to http://www.samplesite.com/design2, design2.css loads. These designs need to stay consistent from page to page at that point.

I apologize for my inability to accurately term what I want to accomplish. Thanks for your help!