Follow the OS setting via prefers-color-scheme (no toggle, no JS). Most
of the site is built on the --od-*/--fg/--bg token layer, so a dark
override of the base color tokens in colors_and_type.css cascades to
every page that uses var(...); inline styles and callouts were already
tokenized or intentionally fixed (e-paper display-color swatches), so
no per-page migration was needed.

- colors_and_type.css: dark values for ink/paper/brand/semantic tokens
  and black-based shadows; swap the header logo mark to a white-outline
  variant via the `content` property so it tracks live scheme changes
  (an <img>-referenced SVG's own media query only re-renders on reload).
- logo-mark.svg: self-adapting outline (keeps the favicon in sync on load).
- logo-mark-dark.svg: static white-outline variant for the live header swap.
- common.css: align the legacy pages' dark palette with the main tokens
  and use dark ink for text on the accent button.

Verified in Chrome (both schemes, live switching): no light-on-dark
leaks and no dark-mode contrast regressions across all pages.