Dynamically change theme of website

I am trying to make a website where the user can select what theme they want to see the website in. My current code allows that if they restart the code, but I want it to be where they can select ‘Earth’ and hit submit, and then the theme changes to ‘Earth’.

As far as I know, this isn’t possible in the current version. My understanding is that this will be available with the new theming engine in v3.