Set up a LAMP (or WAMP, even XAMPP will do it for testing purposes) at your local machine. There you can test and do whatever you want, plus: you dont need to edit the theme files online (slow saving) or even upload them again, and again, and again after changing, to test if it looks good.
For anyone who is interested in looking into this plugin I mentioned: Theme Test Drive.
I'm not a coder, but the part of the coding that handles the actual 'testdrive' is rather short (the rest is admin stuff), so perhaps it can be an example for a ZP version of this plugin...