Dark mode

[svgeesus]

Spun out from this closed pull request.

More work is needed but initial results are promising. I edited a couple of the SVG images, using the same text color as the dark mode stylesheet

The spec gains a light/dark/auto toggler as well

[SamB]

Oh, cool, makes way more sense than trying to add dark mode support to the format 😅.

[Crissov]

That would be dealt with by #24.