UI.16 Layout chrome tone: a layout may be dark, light or follow the theme, with its own chrome tokens

The reference drafting interface is dark chrome around the drawing whatever the OS theme; ours takes --ws-chrome, --ws-line, --ws-blue from the site theme (cc-space light / dark), so a layout cannot ask for a dark frame on a light theme or a paper-white frame on dark. Change: look.tone in the layout format (format v2, UI.6): "theme" (default, as today), "dark", "light"; the engine sets data-ws-tone on the layout's regions and app-workspace.css defines the chrome tokens per tone (sheet, chrome, line, ink, muted, the blue, the tab colours) once, so every block in every layout reads the same six tokens and no block carries a literal colour (a lint over app-workspace.css for hex values outside the token block). The drawing surface keeps the app's own background setting. Settings › Layout shows the tone under the layout name and lets the person override it per layout. Icons follow the tone through UI.15's theme colours.