[developer experience] make the theme picker on local development by default set to Precious Plastic

[pizzaisdavid]

If you're a developer and want to tackle an issue related specifically to precious plastic might get a weird state: the website looks like Precious Plastic but the function isPreciousPlastic will return false.

On local development, this method internally uses a value stored locally in the browser but initially it isn't set.

For example, in Firefox, open http://localhost:3000/academy/intro.html in a private window, then open the storage tab, then click local storage, then click http://localhost:3000.

Only after explicitly selecting a theme is put into storage. This is hard to understand and might trip us new contributors.

initially:

after selecting Precious Plastic in the theme picker at the top:

ideally, on refresh, the theme picker would show which theme is select explicitly:

extra context:
here is a situation where it may have caused confusion: #3292 (comment)