Terminal theme detection and text contrast #1681

Text is unreadable on some terminal themes. A fix (#1152) was closed unmerged.

Items

• Code output still renders as white text on light terminal backgrounds (regression of #704) #736: Code output renders white on light terminal backgrounds (regression of Code output still renders as white text on light terminal backgrounds (follow-up to #617) #704).
• The text boxes are black text in a black box. #809: Text boxes render black text in a black box.

The linked issues hold the original reports and reproductions. Most are closed in favor of this tracker; any child with an open fix PR stays open until that PR lands.