if I want to 'test' another theme without affecting my current site, how do I do it?

Theme Spirit isn't as responsive as I'd like it to be. We migrated our site from WP.com and wonder about going back to the Academica theme since it worked well for us. We'd like to test it without messing up what we've accomplished (which isn't much) with Spirit. What do we need to do to take a theme for a testdrive?