Draft: implement dark mode setting using client side js

Appreciate this wasn't the preferred implementation, but it has the advantage that it is working...

It doesn't seem flashy to me, but maybe browser/client dependent?

Implementing server side has turned into a bit of a rabbit warren. There's no way to manipulate a top-level classlist that I can see from an extension, so that would require a core patch. I am thinking instead it might be best to use asset builder to create a dynamic css file for the vars, based on the setting.

Still working on that.