diff --git a/app/(main)/layout.tsx b/app/(main)/layout.tsx
index 34d9f94a0..828ae4ea4 100644
--- a/app/(main)/layout.tsx
+++ b/app/(main)/layout.tsx
@@ -10,6 +10,7 @@ import {
   matchDomainBranding,
   parseDomainBranding,
   pickRequestHost,
+  type BrandingOverrideKey,
 } from "@/lib/admin/domain-branding";
 import { withBasePath } from "@/lib/browser-navigation";
 import { locales } from "@/i18n/routing";
@@ -37,11 +38,31 @@ const geistMono = Geist_Mono({
   subsets: ["latin"],
 });
 
-export const viewport: Viewport = {
-  width: "device-width",
-  initialScale: 1,
-  viewportFit: "cover",
-};
+// Resolve a branding value for the requesting host: per-domain override first,
+// then the global admin/env/default value (same precedence as app/manifest.ts).
+async function brandedValue(key: BrandingOverrideKey, fallback: string): Promise<string> {
+  const host = pickRequestHost(await headers());
+  const override = matchDomainBranding(
+    host,
+    parseDomainBranding(configManager.get<unknown>("domainBranding", [])),
+  )[key];
+  if (typeof override === "string" && override.length > 0) return override;
+  return configManager.get<string>(key, fallback);
+}
+
+export async function generateViewport(): Promise<Viewport> {
+  await configManager.ensureLoaded();
+  // Chromium uses <meta name="theme-color"> in preference to the manifest's
+  // theme_color when coloring an installed desktop PWA's title bar, so a
+  // hardcoded value here silently overrode the admin setting (#671). Emit the
+  // configured colour instead, honoring per-domain branding like the manifest.
+  return {
+    width: "device-width",
+    initialScale: 1,
+    viewportFit: "cover",
+    themeColor: (await brandedValue("pwaThemeColor", "")) || "#ffffff",
+  };
+}
 
 export async function generateMetadata(): Promise<Metadata> {
   await configManager.ensureLoaded();
@@ -49,15 +70,7 @@ export async function generateMetadata(): Promise<Metadata> {
   // /api/config, app/manifest.ts, and /api/pwa-icon already do. Resolve the
   // request host and prefer its override; fall back to the global
   // admin/env/default value when the host has no favicon override (#585).
-  const host = pickRequestHost(await headers());
-  const domainOverride = matchDomainBranding(
-    host,
-    parseDomainBranding(configManager.get<unknown>("domainBranding", [])),
-  ).faviconUrl;
-  const faviconUrl =
-    domainOverride && domainOverride.length > 0
-      ? domainOverride
-      : configManager.get<string>("faviconUrl", "/branding/Bulwark_Favicon.svg");
+  const faviconUrl = await brandedValue("faviconUrl", "/branding/Bulwark_Favicon.svg");
   // Localize the <head> description to match the UI language; a hardcoded
   // English description is another signal that makes Chrome offer to
   // "translate this page". Resolve the locale from the request path, since this
@@ -97,7 +110,6 @@ export default async function RootLayout({
   return (
     <html lang={locale} dir={getLocaleDirection(locale)} suppressHydrationWarning>
       <head>
-        <meta name="theme-color" content="#ffffff" />
         <meta name="mobile-web-app-capable" content="yes" />
         <meta name="apple-mobile-web-app-capable" content="yes" />
         <meta
diff --git a/lib/__tests__/layout-theme-color.test.ts b/lib/__tests__/layout-theme-color.test.ts
new file mode 100644
index 000000000..1867fd72b
--- /dev/null
+++ b/lib/__tests__/layout-theme-color.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect, vi, beforeEach, type Mock } from 'vitest';
+
+// The root layout pulls in globals.css, next/font and client components purely
+// for rendering; stub them so the test can import generateViewport in isolation.
+// globals.css would otherwise drag Tailwind's PostCSS pipeline into vitest.
+vi.mock('@/app/globals.css', () => ({}));
+vi.mock('next/font/google', () => ({
+  Geist: () => ({ variable: '--font-geist-sans' }),
+  Geist_Mono: () => ({ variable: '--font-geist-mono' }),
+}));
+vi.mock('@/components/service-worker-registration', () => ({ ServiceWorkerRegistration: () => null }));
+vi.mock('@/components/favicon-badge', () => ({ FaviconBadge: () => null }));
+vi.mock('next-intl/server', () => ({
+  getLocale: async () => 'en',
+  getTranslations: async () => (k: string) => k,
+}));
+vi.mock('next/headers', () => ({ headers: vi.fn() }));
+vi.mock('@/lib/admin/config-manager', () => ({
+  configManager: { ensureLoaded: vi.fn(async () => {}), get: vi.fn() },
+}));
+
+import { generateViewport } from '@/app/(main)/layout';
+import { headers } from 'next/headers';
+import { configManager } from '@/lib/admin/config-manager';
+
+const mockHeaders = headers as unknown as Mock;
+const mockGet = configManager.get as unknown as Mock;
+
+/** Config values keyed by admin key; anything unset falls back to the caller's default. */
+function withConfig(values: Record<string, unknown>) {
+  mockGet.mockImplementation((key: string, fallback: unknown) =>
+    key in values ? values[key] : fallback,
+  );
+}
+function onHost(host: string) {
+  mockHeaders.mockResolvedValue(new Headers({ host }));
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  onHost('mail.example.com');
+  withConfig({});
+});
+
+describe('root layout generateViewport - theme color (#671)', () => {
+  it('emits the admin-configured theme color, not a hardcoded white', async () => {
+    withConfig({ pwaThemeColor: '#1a1b26' });
+    expect((await generateViewport()).themeColor).toBe('#1a1b26');
+  });
+
+  it('prefers a per-domain branding override for the requesting host', async () => {
+    withConfig({
+      pwaThemeColor: '#1a1b26',
+      domainBranding: [{ host: 'other.example.com', pwaThemeColor: '#7c3aed' }],
+    });
+    onHost('other.example.com');
+    expect((await generateViewport()).themeColor).toBe('#7c3aed');
+
+    onHost('mail.example.com');
+    expect((await generateViewport()).themeColor).toBe('#1a1b26');
+  });
+
+  it('falls back to white when nothing is configured', async () => {
+    withConfig({ pwaThemeColor: '' });
+    expect((await generateViewport()).themeColor).toBe('#ffffff');
+  });
+
+  it('keeps the viewport sizing fields intact', async () => {
+    const viewport = await generateViewport();
+    expect(viewport.width).toBe('device-width');
+    expect(viewport.initialScale).toBe(1);
+    expect(viewport.viewportFit).toBe('cover');
+  });
+});