Plot’s default background: white often isn’t a great default

[mbostock]

The background: white is nice for generating opaque screenshots when using Download PNG on Observable, but it makes it harder to use Plot in another web application, say with a dark theme—especially because Plot’s foreground color defaults to currentColor and not black. When Plot supports themes #630 (configurable defaults), it might makes sense to have the default background: white only apply on Observable. Perhaps we can have a workaround in the interim, say by looking at location.origin.