While modifying new site, try out and switch between themes?

ChancyRat

I've just set up a test theme with customizations, including for example a navbar with dropdowns for all sub folders and galleries. Now I would like to try out a new theme (some of the colors, mainly), without having to re-do all the customizations (like the nav bar). I see the "active theme" field. How do I add new theme choices to this list and then switch between them as I customize?