Plot.boxX default fill should be responsive with dark mode

[Fil]

The default fill for boxX is #ccc, which is way too light in dark mode (when --plot-background is dark and currentColor is light).

Should we address this by "hardcoding" a new color in box.js, say color-mix(in oklab, var(--plot-background), currentColor 80%), or by introducing an intermediate --color80? The only other "hardcoded" colors are

• stroke = "#555" in treeLink (which would be --color33 in my scheme)
• positiveFill = "#3ca951", negativeFill = "#4269d0", in differenceY (but that one is ok I think)