no flashing when reloading in light or dark mode

[gordonwoodhull]

fixes #1325

Introduces a new script before the body which runs immediately, enabling or disabling dark mode.

Also, both light and dark stylesheets are loaded at once, and then one immediately disabled, instead of dark starting as rel="prefetch", which was one of the causes of flashing.

Submitting as a draft for three reasons:

1. It's a pretty big change to the structure of the HTML, would appreciate folks giving it a look-over and/or testing.
2. It does not degrade nicely when JavaScript is disabled - want to investigate if there is an alternative that does load the dark stylesheet eagerly but disables it. ¹
3. I'm out next week and don't want to check in this big a change before I go.

Footnotes

1. Worst case, it does work to load the light stylesheet after the dark stylesheet, with a few fixes to the JS that currently depend on stylesheet order. ↩