Changing current theme via URL (GET)

I’m wanting to develop a new theme for my blog, but I don’t want to take down the current site or have the changes be done on a live version of the site as it’s likely to take quite some time.

So, what I’m looking for is a way to define the current theme using the URL (eg: /blog/?theme=newone). Is there any way to do this with the current WordPress code, or perhaps a simple way to patch or plug in this functionality?

Thanks for any help!