Dark mode toggle

The default is an icon button for a navbar slot: a moon in light mode, a sun in dark. It reads the theme from the visitor's saved choice, then the cookie, then the system setting, and saves a change to both so the next page paints in the right theme first time. Where the browser supports it, the new theme reveals as a circle from the button. Try it; it switches this whole page.

Labelled is a full-width menu row with the icon and "Dark mode" or "Light mode", for the Portal user menu.