the very simplest thing to do would be to install apache/php/mysql on your home computer. The WAMP package, available at http://www.wampserver.com/en/index.php is completely free, and the install is almost brainless. Once installed, you simply copy your WordPress package over to the proper directory within WAMP, and it should start up, after you do the required wordpress database setup using phpmyadmin (which is included).
After setup, you simply get WAMP booted, browse to your new wordpress testbed directory, and then start up your editor in another window, to make your changes to your theme. Simply go back and forth between your server window (what the world would see if you moved the WP files online), and your edit window...be sure to make ONE change at a time, so you know exactly what broke when you break it in your editing mode.
Trial and error will get you up to speed. Until someone else writes a SPECIFIC WP Theme viewer, this will get you going within an hour, of doing exactly what you want to do.