feat: theme toggle

CLAUDE.md

@@ -53,7 +53,12 @@ unreachable — the site must never 500 because Strapi is down.
    The site ships RTL Arabic; physical directions silently break it.
    Check: `grep -rnE "\b(pl|pr|ml|mr|left|right)-[0-9]" apps/web/app` → nothing meaningful.
 3. **Surfaces, not a theme.** `:root` *is* the dark surface; `data-surface="light"` flips a
-   subtree and repaints itself. Do not add a global theme toggle or a `dark:` variant.
+   subtree and repaints itself. Never add a second theme system or a `dark:` variant. **The
+   one exception, accepted deliberately:** the nav's theme toggle flips *which* surface `<html>`
+   stands on (dark default ↔ light), persisted via the `sharper-theme` cookie read in the root
+   loader and set by the root action — SSR-rendered so the first byte and first paint always
+   match. The mechanism stays surfaces-only: the toggle just re-roots them. `--surface-nav`
+   must keep a value in BOTH surface blocks.
 4. **Figma is READ ONLY.** Only `get_design_context`, `get_screenshot`, `get_metadata`,
    `get_variable_defs`. Never call anything that writes (`use_figma`, `generate_figma_design`,
    `create_new_file`, `upload_assets`, shaders, plugins, …). Before the first

apps/web/app/components/ui/Navigation/Navigation.tsx

@@ -10,6 +10,7 @@ import {
 import { Brand } from "./Brand";
 import { DesktopNav } from "./DesktopNav";
 import { MobileNav } from "./MobileNav";
+import { ThemeToggle } from "./ThemeToggle";
 import { getDefaultNavItems, getNavigationStrings } from "./nav-items";
 import { cx } from "./styles";
 import type { NavItem, NavigationStrings, NavSurface } from "./types";
@@ -53,9 +54,10 @@ export interface NavigationProps {
  * and the whole tree moves into a full-screen drawer.
  *
  * EU VISIBILITY: the updated design has no funding badge in the bar - the
- * Actions slot holds the language globe alone. The mandatory "Funded by the
+ * Actions slot holds the language globe and the theme toggle. The mandatory
- * European Union" lockup is carried by <Footer />, which renders on every page,
+ * "Funded by the European Union" lockup is carried by <Footer />, which
- * and by the mobile drawer; it is not dropped from the site.
+ * renders on every page, and by the mobile drawer; it is not dropped from the
+ * site.
  */
 export function Navigation({
   locale = "en",
@@ -148,6 +150,11 @@ export function Navigation({
             </div>
           )}
 
+          {/* The theme control sits with the other actions at every breakpoint:
+              after the (desktop-only) language cluster on wide screens, beside
+              the hamburger on a compact bar. */}
+          <ThemeToggle locale={locale} />
+
           <MobileNav
             items={navItems}
             pathname={pathname}

apps/web/app/components/ui/Navigation/ThemeToggle.tsx

@@ -0,0 +1,111 @@
+import { useMemo } from "react";
+import { useFetcher, useRouteLoaderData } from "react-router";
+
+import { createTranslator } from "~/i18n/t";
+import type { AppTheme } from "~/lib/theme";
+
+import type { Locale } from "../LanguageSwitcher";
+import { cx, FOCUS_RING, HOVER_TRANSITION } from "./styles";
+
+/** The document surface the root loader currently paints on <html>. */
+interface RootLoaderData {
+  theme?: AppTheme;
+}
+
+/** A minimal sun, for "switch to light". Decorative - the label calls it. */
+function SunIcon() {
+  return (
+    <svg
+      width="18"
+      height="18"
+      viewBox="0 0 24 24"
+      fill="none"
+      stroke="currentColor"
+      strokeWidth="2"
+      strokeLinecap="round"
+      strokeLinejoin="round"
+      aria-hidden="true"
+    >
+      <circle cx="12" cy="12" r="4" />
+      <path d="M12 2v2M12 20v2M4.93 4.93l1.41 1.41M17.66 17.66l1.41 1.41M2 12h2M20 12h2M6.34 17.66l-1.41 1.41M19.07 4.93l-1.41 1.41" />
+    </svg>
+  );
+}
+
+/** A minimal crescent moon, for "switch to dark". Decorative. */
+function MoonIcon() {
+  return (
+    <svg
+      width="18"
+      height="18"
+      viewBox="0 0 24 24"
+      fill="none"
+      stroke="currentColor"
+      strokeWidth="2"
+      strokeLinecap="round"
+      strokeLinejoin="round"
+      aria-hidden="true"
+    >
+      <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z" />
+    </svg>
+  );
+}
+
+export interface ThemeToggleProps {
+  /** Reading locale, for the accessible label. */
+  locale: Locale;
+  className?: string;
+}
+
+/**
+ * The site's one global theme control: sun in dark mode, moon in light.
+ *
+ * It renders the target of the next click - the label and the icon describe
+ * what a press DOES ("Switch to light theme"), never the current state - and
+ * posts that target to the root action at `/`. The action persists the
+ * `sharper-theme` cookie, the loader re-runs and the shell repaints
+ * `data-surface` on <html>, so the whole tier-2 palette re-resolves. Because
+ * the current theme is read off the root loader (which the server fed from
+ * the cookie), the SSR markup and the first client render always agree.
+ */
+export function ThemeToggle({ locale, className }: ThemeToggleProps) {
+  const fetcher = useFetcher();
+  const rootData = useRouteLoaderData<RootLoaderData>("root");
+  const t = useMemo(() => createTranslator(locale), [locale]);
+
+  const theme: AppTheme = rootData?.theme ?? "dark";
+  const targetTheme: AppTheme = theme === "light" ? "dark" : "light";
+  const label =
+    targetTheme === "light" ? t("nav.themeToLight") : t("nav.themeToDark");
+
+  return (
+    <fetcher.Form action="/" method="post">
+      <input type="hidden" name="theme" value={targetTheme} />
+      <button
+        type="submit"
+        aria-label={label}
+        title={label}
+        className={cx(
+          "inline-flex size-9 shrink-0 cursor-pointer items-center justify-center",
+          "rounded-control text-[var(--text-secondary)]",
+          "hover:text-[var(--text-primary)]",
+          HOVER_TRANSITION,
+          FOCUS_RING,
+          className,
+        )}
+      >
+        {targetTheme === "light" ? <SunIcon /> : <MoonIcon />}
+      </button>
+    </fetcher.Form>
+  );
+}
+
+export default ThemeToggle;
+
+/* ---------------------------------------------------------------------------
+   USAGE
+   ---------------------------------------------------------------------------
+   // In the nav actions cluster, between the language control and the
+   // hamburger. Reads the theme from the root loader on its own.
+   <ThemeToggle locale={locale} />
+   ------------------------------------------------------------------------ */

apps/web/app/i18n/messages/ar.json

@@ -17,6 +17,8 @@
     "closeMenu": "إغلاق القائمة",
     "homeLinkLabel": "الصفحة الرئيسية لمشروع SHARP-ER",
     "languageMenu": "اللغة",
+    "themeToLight": "التبديل إلى المظهر الفاتح",
+    "themeToDark": "التبديل إلى المظهر الداكن",
     "skipToContent": "تخطَّ إلى المحتوى",
     "sections": {
       "home": "الرئيسية",

apps/web/app/i18n/messages/en.json

@@ -17,6 +17,8 @@
     "closeMenu": "Close menu",
     "homeLinkLabel": "SHARP-ER home",
     "languageMenu": "Language",
+    "themeToLight": "Switch to light theme",
+    "themeToDark": "Switch to dark theme",
     "skipToContent": "Skip to content",
     "sections": {
       "home": "Home",

apps/web/app/lib/theme.ts

@@ -0,0 +1,33 @@
+/* ============================================================================
+   SITE THEME
+   ----------------------------------------------------------------------------
+   The one global theme decision the site makes. The whole tier-2 palette is
+   still per-surface (see app/styles/semantic.css); this simply lets a reader
+   flip WHICH surface the document stands on, `:root` dark by default. The
+   preference travels as the `sharper-theme` cookie so the server renders the
+   same surface the next page will paint - no flash-of-wrong-theme, no
+   hydration mismatch.
+   ========================================================================= */
+
+export type AppTheme = "dark" | "light";
+
+export const THEME_COOKIE = "sharper-theme";
+
+/** Default when no cookie is present. `:root` is the dark surface. */
+export const DEFAULT_THEME: AppTheme = "dark";
+
+/** Parse the cookie header into a theme. Anything unknown resolves to dark. */
+export function readTheme(cookieHeader: string | null): AppTheme {
+  if (!cookieHeader) return DEFAULT_THEME;
+  const pair = cookieHeader
+    .split(";")
+    .map((part) => part.trim())
+    .find((part) => part.startsWith(`${THEME_COOKIE}=`));
+  const value = pair?.split("=")[1];
+  return value === "light" ? "light" : value === "dark" ? "dark" : DEFAULT_THEME;
+}
+
+/** The Set-Cookie value that persists a theme choice for a year. */
+export function themeCookie(theme: AppTheme): string {
+  return `${THEME_COOKIE}=${theme}; Path=/; Max-Age=31536000; SameSite=Lax`;
+}

apps/web/app/root.tsx

@@ -4,7 +4,9 @@
    `Layout` renders the whole document, which is why <html lang> and <html dir>
    are correct in the FIRST SSR byte: the locale is read from the URL, not from
    a cookie, a context provider or an effect, so the server and the first
-   client render cannot disagree.
+   client render cannot disagree. The theme surface is the one deliberate
+   exception to "no cookies" - it is a cookie read in the loader, but because
+   the loader is what the server renders, the first byte still matches.
 
    The header and the footer live here rather than in the ":lang" layout so the
    ErrorBoundary keeps its chrome: a 404 is a normal page of the site, with the
@@ -43,6 +45,7 @@ import {
 } from "~/lib/route-handle";
 import { alternateUrls, buildMeta, canonicalUrl } from "~/lib/seo";
 import { buildFooterLinkGroups, buildNavItems } from "~/lib/site-navigation";
+import { DEFAULT_THEME, readTheme, themeCookie, type AppTheme } from "~/lib/theme";
 
 /**
  * Last-resort origin for the canonical and hreflang links. The root loader
@@ -80,9 +83,31 @@ export function loader({ request }: Route.LoaderArgs) {
     dir: dirFor(locale),
     pathname,
     siteUrl: getSiteUrl(request),
+    /* Which surface the document stands on. Read from the theme cookie so the
+       server paints the same surface the reader asked for - the browser never
+       guesses, so there is no flash-of-wrong-theme and no hydration mismatch.
+       `:root` is dark, so the absence of a cookie is a dark first visit. */
+    theme: readTheme(request.headers.get("Cookie")),
   };
 }
 
+/**
+ * The root action is the theme toggle's destination (the nav posts here at
+ * `/`). It validates the requested theme and persists it with Set-Cookie; the
+ * loader re-runs and the shell repaints the `data-surface` on <html>.
+ */
+export async function action({ request }: Route.ActionArgs) {
+  const formData = await request.formData();
+  const requested = formData.get("theme");
+  const theme: AppTheme =
+    requested === "light" ? "light" : requested === "dark" ? "dark" : DEFAULT_THEME;
+
+  return new Response(null, {
+    status: 204,
+    headers: { "Set-Cookie": themeCookie(theme) },
+  });
+}
+
 function pathnameLocale(pathname: string): Locale {
   const first = pathname.replace(/^\/+/, "").split("/")[0]?.toLowerCase();
   return first === "ar" ? "ar" : first === "en" ? "en" : DEFAULT_LOCALE;
@@ -126,13 +151,19 @@ export function Layout({ children }: { children: ReactNode }) {
   const navItems = useMemo(() => buildNavItems(locale), [locale]);
   const footerLinkGroups = useMemo(() => buildFooterLinkGroups(locale), [locale]);
 
-  const navSurface = useRouteHandleValue(readNavSurface) ?? "dark";
+  /* The theme cookie decides which surface the DOCUMENT stands on. The header
-  const footerSurface = useRouteHandleValue(readFooterSurface) ?? "dark";
+     and footer follow it, unless the route's handle forces one (`navSurface`
+     / `footerSurface`) - so a page can still draw a light nav on a dark
+     surface in either theme. */
+  const rootData = useRouteLoaderData<typeof loader>("root");
+  const theme: AppTheme = rootData?.theme ?? DEFAULT_THEME;
+
+  const navSurface = useRouteHandleValue(readNavSurface) ?? theme;
+  const footerSurface = useRouteHandleValue(readFooterSurface) ?? theme;
 
   /* Canonical and hreflang are emitted HERE, not from a route's `meta`: a leaf
      `meta` replaces its parents' wholesale, so a page that writes its own
      would silently drop them. `handle.canonicalPath` is the escape hatch. */
-  const rootData = useRouteLoaderData<typeof loader>("root");
   const siteUrl = rootData?.siteUrl ?? FALLBACK_SITE_URL;
   const canonicalPath = useRouteHandleValue(readCanonicalPath) ?? pathname;
   /* A detail route emits its own pair from the counterpart's real Strapi slug
@@ -142,7 +173,7 @@ export function Layout({ children }: { children: ReactNode }) {
   const ownsAlternates = useRouteHandleValue(readSelfAlternates) ?? false;
 
   return (
-    <html lang={locale} dir={dir} data-surface="dark">
+    <html lang={locale} dir={dir} data-surface={theme}>
       <head>
         <meta charSet="utf-8" />
         <meta name="viewport" content="width=device-width, initial-scale=1" />

apps/web/app/styles/README.md

@@ -144,6 +144,18 @@ Because both surface blocks define the *same* token names, a component written
 against `--text-primary` is automatically correct on either one - which is why
 components must never reach past a role to a primitive.
 
+**The one global exception, accepted deliberately - the theme toggle.** The
+nav's sun/moon control (Navigation/ThemeToggle.tsx) lets a reader re-root the
+surface the whole document stands on: the root action persists the
+`sharper-theme` cookie, the root loader returns it and `<html
+data-surface={theme}>` follows, so a light-theme visit stands on
+`[data-surface="light"]` and every token re-resolves. The mechanism is still
+surfaces-only - the toggle never repaints anything itself, it just chooses
+which pole `:root` behaviour applies to - and the header/footer follow the
+theme unless a route's `handle` forces a surface. `--surface-nav` (the
+translucent bar) must stay defined in BOTH surface blocks, or a light-theme
+nav resolves against the dark block.
+
 Utility variants `surface-dark:` and `surface-light:` exist for the rare case a
 utility (not a token) has to differ, and `rtl:` / `ltr:` for direction. Prefer
 CSS logical properties (`ps-`, `pe-`, `ms-`, `me-`, `start-`, `end-`) so RTL

apps/web/app/styles/semantic.css

@@ -17,6 +17,13 @@
    :root and [data-surface="dark"] share one block; [data-surface="light"]
    comes afterwards so it wins on the same element, and proximity makes the
    nearest data-surface ancestor win for any subtree.
+
+   THEME TOGGLE (the declared exception to "no global theme"). The nav's
+   theme control flips WHICH surface the DOCUMENT stands on: the root action
+   persists `sharper-theme`, the root loader returns it, and <html
+   data-surface> follows - so light mode means the whole site stands on
+   [data-surface="light"] and every token re-resolves, including the nav and
+   footer, which follow the theme unless a route's handle forces a surface.
    ========================================================================= */
 
 /* ---------------------------------------------------------------------------
@@ -193,6 +200,9 @@
   --surface-danger-subtle: var(--color-red-100);
   --surface-inverse: var(--color-neutral-900);     /* figma background/inverse #0b0b0a */
   --surface-footer: var(--color-white);            /* figma 140:7812, the light footer */
+  /* The light bar floats the same way the dark one does - the ground colour at
+     90% with a blur behind it, so content shows through as it scrolls under. */
+  --surface-nav: color-mix(in srgb, var(--color-white) 90%, transparent);
   --surface-selection: var(--color-green-200);
 
   /* --- text ------------------------------------------------------------- */