[css-box-3] info panels render with black text on black background, because the spec html has incomplete color-scheme-friendliness CSS

[dholbert]

The css-box-3 draft has got some "black text on black background" sorts of issues, if you view it with a dark color-scheme preference -- specifically in info-panels. Screenshot of what happens when I click on the "origin box" term in https://drafts.csswg.org/css-box-3/#valdef-box-view-box :

This is in Firefox with a dark color-scheme preference selected in preferences. The issue is also reproducible in Chrome/Safari on macOS with dark color-scheme in system settings.

[dholbert]

The issue seems to be that this particular spec draft's HTML has incomplete color-scheme css. Most/all other spec drafts seem fine -- e.g. the css-box level 4 spec, and the css-borders-4 spec and css-grid-1 specs render with a black background and light text when viewed in this configuration, and their info boxes seem to render with light text over a dark gray background, like this (which looks fine):

[dholbert]

(CC @fantasai since she's listed as the spec editor for css-box-3.)