Using history.pushstate for each theme is really obnoxious; after clicking a few radio buttons, I had to hit "back" a dozen times to get back to this page.
Hi! Author of the project here... Sorry you found it disturbing, the intention was to encode the selected theme in the URL so that it can be easily linked to, etc. Maybe I should look into trying to do that with replaceState. Having said that, the code is open-source and PRs are always welcome. ;)
replaceState would be quite welcome! It's good to allow specific themes to be linkable, but it's bad to pollute the history when you click radio buttons, IMO.
Other than that, though, good work. :)
Many browsers allow you to long-click the back/forward buttons to get a list of pages, and you can select just where you want to go back to.