This assumes that you have the folders for the switching prepared, and you want to verify that it works.

1. Make sure that the templates in the various folders have something different - it can just be a note or something - to ensure you can tell which one you are seing. 
2. Set the page theme to a theme that has one of the desired CSS-Frameworks and reload the page. You should now see that the correct output was generated. 
3. Set the page theme to another theme targeting another framework - you should see that it works.

Troubleshooting

If you keep seing the same same output you usually made one of these mistakes:

1. Your templates are so similar that you can't tell which one was loaded
2. Your templates don't have matching names (that's required)
3. Your theme doesn't have a koi.json, so it doesn't publish what CSS-framework it's using

If all else fails, look at the insights. You should see entries about Polymorph - search for these to see what the engine was doing.