[wip] demo day/night with radios

[bollwyvl]

Hey, folks! Glad to see the theme variable stuff got in. It looks like it still fights with bootstrap a fair amount, but otherwise cool stuff!

As suggested elsewhere, wrapping all themeable content in a wrapper div unlocks the hackish, but usable, CSS-only, flicker-free day/night switch, powered by the ~ selector.

This PR does a very minimal port of some of those techniques, up to where it's at least half-way readable, but has a lot of rough edges in the implementation.

• I still really recommend doing a CSS variable prefix, e.g. --pst-
• doing the JS part to write to localStorage right in the DOM, outside webpack... got no time for MBs to load
  • this can be cleaner, though, and should scale to other modes, and/or ones with more than two values
• defining the CSS variables probably needs an overhaul vs what i've done
  • the selector implementation probably shouldn't leak out
• currently doesn't handle code syntax highlighting Would night mode *only* in code cells be useful? #204
  • this is a whole, separate ball of wax, but having some answer is probably important enough to block any serious consideration of this at this point

closes #458

[bollwyvl]

hah, well, not going to fight with the linter right now, but here's a screenshot of my bad theme colors:

[choldgraf]

Hey all - I created an issue to track this one so that we don't lose track of it, as I think it is quite important! (#458). @bollwyvl what would you like to do with this PR? Steward it to completion? Re-start now that some of the underlying HTML/CSS has changed?

By the way - you had a few nice suggestions in there as well that might be out of scope for this PR (e.g. CSS prefixes). Do you think some of them deserve their own issues?

(ps: I super appreciate the work that you had put into these PRs in the past and present!)

[12rambau]

Should it be considered solved by #540 ?
I used this PR as a starting point but I wasn't able to start from it as it's now to far away from master.