I also just started playing around with this. My understanding is that you can have many themes. Each theme would be in its own folder (same location where you put the first one). Then you would zip that folder up and add as a new extension.
Then they all should show up in the list of themes in your settings for the app.
You could just try this with the one you have already created. Copy the folder and give it a new name. After you load it you should see both of them in the list even though they would have the same affect.