In addition to the color scheme changing based on the system setting, enable
setting the color scheme explicitly by adding a color-scheme="light | dark" attribute
on the <html> element.
Doing this was a bit tricky. I originally implemented the grayscale ramp by
reversing it when prefers-color-scheme: dark. This was convenient, but meant that
setting the color scheme explicitly didn't work.
Along the way I discovered the light-dark() CSS function. Deploy that as the preferred
style if the browser supports it. Otherwise, fall back on the prefers-color-scheme
media queries. This function only works if color-scheme: light dark is set on the
:root element.
Draw only one hash for headings, and place them in the gutter in wide layouts,
or trailing the heading text in narrow layouts. Do this with grid in the wide
version, and flexbox in the narrow layout.
Add a --text-color-light CSS variable for content that should be lighter than
--text-color-secondary.
