Context
ADR-0042 gave the site one light scheme and had the second-brain map at /map/ draw its light palette. On the light page colour the graph lost depth: the nodes and the links between them read as thin marks on paper. Stefan Coetzee asked the same day for a black background behind the graph only.
Decision
The canvas is dark graph art. The script draws the canvas on the top bar's colour, read from --mb-bar-bg in design/tokens.css, so the bar and the graph read as one dark field and the colour lives in one place. The nodes take the bright colours and the labels the light ink they had on the dark canvas before ADR-0042. The page's own background behind the canvas is the same colour, so nothing light shows before the canvas draws.
Everything a person reads in a box stays light. The panel, the hover tip and the preview sheet are solid light boxes (--mb-surface-raised with --ink), as on every page; text that sits on the dark background itself, such as the page's no-script note, takes the bar's text colours. The legend dots in the panel carry a thin outline mixed from --mb-bar-bg, because the bright node colours alone are faint on white. The stamp at the bottom right sits on the canvas and uses the bar's text colours (--mb-bar-dim, --mb-bar-soft).
The contrast floor holds on the canvas. The stamp reaches 6.3:1, its link 11.6:1, the node labels about 11.6:1 against the canvas, and every node colour 5.1:1 or more.
This replaces the map paragraph of ADR-0042; the rest of ADR-0042 stands. The dark canvas joins the top bar and the report theme's title band as dark chrome on a light site.
Consequences
The map looks the same in every system setting, with the graph on dark and the controls on light. A change to the top bar's colour changes the canvas with it; the inks drawn on it (labels, hover, focus, selection) are fixed values chosen for this dark, so such a change needs them rechecked. The second-brain image in the front page's title band was already a dark rendering of the same graph, so the front page and the map now agree.