Suppose I have made a clone site of my main website using the sharing feature in order to do some theme development. I've made some changes to the clone site. How, now, do I compare my edited clone site to the live site, and copy my changes (or a selection of them) to the live site?
If you want to compare templates of separate websites, I'd recommend using an external tool like Diff Checker.
Instead of cloning your site and theme to test changes, you could also connect Dropbox to your nation. This integration would allow you to work on your live theme without the concern publishing unwanted changes, access a version history of all of your files, and preview the changes you make as you go.
Complete and current information on how to use NationBuilder is available in the documentation section.