diff --git a/app/src/App.js b/app/src/App.js
index c3299d98..088cfa95 100644
--- a/app/src/App.js
+++ b/app/src/App.js
@@ -87,10 +87,12 @@ function Root() {
     </QueryProvider>
 }
 
-const router = createBrowserRouter(createRoutesFromElements(<Route
+export const routes = createRoutesFromElements(<Route
     path='/'
     element={<Root/>}
-    errorElement={<PageNotFound/>}
+    // Root (and its ThemeProvider) is not rendered when Root itself fails, so the error page
+    // brings its own theme; Mantine components cannot render without one.
+    errorElement={<ThemeProvider><PageNotFound/></ThemeProvider>}
 >
     <Route index element={<ErrorBoundary><DashboardPage/></ErrorBoundary>}/>
     <Route path='search/*' element={<ErrorBoundary><DashboardPage/></ErrorBoundary>}/>
@@ -121,7 +123,11 @@ const router = createBrowserRouter(createRoutesFromElements(<Route
     <Route path='repos/*' element={<ErrorBoundary><ReposRoute/></ErrorBoundary>}/>
     <Route path='files/*' element={<ErrorBoundary><FilesRoute/></ErrorBoundary>}/>
     <Route path='flasher/*' element={<ErrorBoundary><FlasherRoute/></ErrorBoundary>}/>
-</Route>));
+    {/* Unknown URLs render inside Root, with the navigation bar and theme. */}
+    <Route path='*' element={<PageNotFound/>}/>
+</Route>);
+
+const router = createBrowserRouter(routes);
 
 export default function App() {
     useEventsInterval();
diff --git a/app/src/App.test.js b/app/src/App.test.js
new file mode 100644
index 00000000..f7e78352
--- /dev/null
+++ b/app/src/App.test.js
@@ -0,0 +1,52 @@
+import React from 'react';
+import {render, screen} from '@testing-library/react';
+import {createMemoryRouter, RouterProvider} from 'react-router';
+import {routes} from './App';
+import {StatusProvider} from './hooks/customHooks';
+import {FileWorkerStatusProvider} from './contexts/FileWorkerStatusContext';
+import {MediaContextProvider} from './contexts/contexts';
+
+// Jest does not transform CSS inside node_modules (see transformIgnorePatterns).
+jest.mock('@mantine/core/styles.css', () => ({}));
+jest.mock('@mantine/notifications/styles.css', () => ({}));
+// Routes this test does not visit, whose libraries jsdom/Jest cannot load (maplibre needs
+// browser APIs; esptool-js is ESM-only).
+jest.mock('./components/Map', () => ({MapRoute: () => null}));
+jest.mock('./components/Flasher', () => ({FlasherRoute: () => null}));
+
+describe('unknown routes', () => {
+    beforeEach(() => {
+        // resetMocks clears setupTests' implementation; the theme and media providers read it.
+        window.matchMedia.mockImplementation(query => ({
+            matches: false,
+            media: query,
+            addListener: jest.fn(),
+            removeListener: jest.fn(),
+            addEventListener: jest.fn(),
+            removeEventListener: jest.fn(),
+        }));
+        global.fetch = jest.fn(() => Promise.resolve({
+            ok: true,
+            status: 200,
+            headers: {get: () => 'application/json'},
+            json: () => Promise.resolve({tags: []}),
+            text: () => Promise.resolve('{}'),
+        }));
+    });
+
+    it('renders Page Not Found inside the app layout', async () => {
+        const router = createMemoryRouter(routes, {initialEntries: ['/no-such-page']});
+        // The providers App wraps around the router.
+        render(<StatusProvider>
+            <FileWorkerStatusProvider>
+                <MediaContextProvider>
+                    <RouterProvider router={router}/>
+                </MediaContextProvider>
+            </FileWorkerStatusProvider>
+        </StatusProvider>);
+
+        expect(await screen.findByText('Page Not Found!')).toBeInTheDocument();
+        // Rendered inside Root's layout (the footer comes from Root), not as a bare error page.
+        expect(screen.getByText('Donate')).toBeInTheDocument();
+    });
+});