In this screencast we add a new plugin to the site that gives us the ability to switch the themes on the live site, but only so that we (as admins) can see it, not regular visitors.
Before we add it, we look at the status of our project in Tower. We have a number of files to commit that have changed. We haven't been showing as much as we should have about using Git in this project. But we have been doing a good job! Every time we make a few related changes, we pop over to Tower and commit the changes in little batches. We do that here to clean up the status now.
We hop over to the live site and see that a number of plugins need updating. We could do that on the live site - but that's not how we work - we work locally and push live. This makes it easier anyway. For us, we have the live site permissions set such that the one-click install doesn't work without providing FTP information anyway. But locally one-click install works great. So we simply do the one-click installs locally then push up the changes through Git.
Next we go snag and install the Theme Switch and Preview plugin. It's a little old, but it works fine. After it's pushed, we go activate it on the live site.
Now we have the ability to look at the live site with our new theme, which is the ultimate test! We can even have links to share with others to allow them to see the new theme too, so we could pull in some folks to help with testing if needed.