Is there some automated way or script I can use to find and preferably delete all the unused CSS on my site?
If you use Firefox, you can install a plugin called Dust-Me Selectors and use that to locate unused CSS. Then, simply strip the stuff you don't need out of your stylesheet(s).
There is not much to "automate" such a process. The problem is that e.g. Google Page Speed addon can tell you what CSS code is "unused", but the addon is not correct. What the addon tells you is that the CSS code that is "unused" is only not necessary to render the site after it got loaded. That's why I delete unused CSS by hand, I know what I have added there and I know which code can be deleted.
Automation is dangerous. If it scans a page and returns a handful of classes that aren't used on that specific page, it doesn't necessarily mean it's not in use elsewhere.
Google Page Speed tells you the unused CSS ON THE PAGE YOUR VIEWING. Not the entire site. So be careful that your 'unused' styles are not needed on sub pages.