Themes: five presets, chosen in admin, applied live to every dashboard

Server: one THEMES list; `theme` is read from the data directory's proddash.json like port/adminPasscode; GET /api/theme is public (dashboards need it before anyone logs in) and returns { theme, themes: [{ id, name, colors: { bg, panel, accent } }] }; POST /api/admin/theme { theme } validates against the list, writes the key into proddash.json keeping every other key, updates the in-memory value and broadcasts a `theme` shell event over /api/events.

style.css: four html[data-theme] variable sets appended at the end — Harbor (navy/cyan), Graphite (charcoal/blue), Ember (warm black/orange) and Daylight (light, for daytime FOH) — each swapping exactly the eight palette variables; :root stays Booth, the default.

shell.js: a self-contained theme section — the last-known theme (localStorage proddash:theme) goes on synchronously, then the server's answer, then live changes via the SSE event; one listener line added in subscribeEvents. Tile-configured colours are set on the tile and still win.

README: a Themes subsection under Configure and the `theme` row in the proddash.json key table.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -99,6 +99,7 @@ with this machine's shell settings — the same keys as the checked-in
 | --- | --- |
 | `port` | Port ProdDash serves on (default `24500`). The macOS launcher writes this one, so the port it is set to holds however ProdDash is started. |
 | `adminPasscode` | If set, the admin page asks for this shared passcode. Leave `""` for none — it's a plain-HTTP LAN tool. |
+| `theme` | The dashboard palette: `booth` (default), `harbor`, `graphite`, `ember` or `daylight`. The admin page's **Theme** section writes this one — see [Themes](#themes). |
 
 Environment variables override both files: `PORT`, `PRODDASH_PASSCODE`, and
 `PRODDASH_DATA_DIR` to put the data directory somewhere else (a relative path
@@ -111,6 +112,22 @@ so and falls back to the app's `config/` folder.
 directory, so nothing has to be re-entered. The old files are left in place and
 ignored from then on.
 
+### Themes
+
+The whole dashboard — every tile, on every browser — draws with one palette,
+chosen in the admin page's **Theme** section and switched live: pick a swatch
+and open dashboards change without a reload. Five presets ship: **Booth**
+(the default dark, mint accent), **Harbor** (deep navy, cyan), **Graphite**
+(neutral charcoal, electric blue), **Ember** (warm broadcast black, orange)
+and **Daylight**, a light palette for a front-of-house desk in daytime. Each
+keeps the status colours apart — the accent is never near the danger red or
+the warning amber — and muted text readable in a dark booth. The choice is
+stored as `theme` in the data directory's `proddash.json` (table above), so
+it holds across restarts and updates; a browser remembers the last theme it
+saw and applies it before the server answers, so nothing flashes. Modules
+style with the shell's colour variables, which is why they follow; a colour
+set on a tile itself (a PCO Plan header colour, say) is deliberately kept.
+
 ## Installing modules and updating
 
 Everything below happens in the **admin page**; ProdDash talks to its GitHub