♻️(frontend) get theme from config endpoint

We centralized the configuration on the backend
side, it is easier to manage and we can change
the configuration without having to rebuild the
frontend.
We now use the config endpoint to get the theme,
we refacto to remove the frontend env occurences
and to adapt with the new way to get the theme.

src/frontend/apps/e2e/__tests__/app-impress/config.spec.ts

@@ -56,4 +56,25 @@ test.describe('Config', () => {
 
     expect((await consoleMessage).text()).toContain(invalidMsg);
   });
+
+  test('it checks that theme is configured from config endpoint', async ({
+    page,
+  }) => {
+    const responsePromise = page.waitForResponse(
+      (response) =>
+        response.url().includes('/config/') && response.status() === 200,
+    );
+
+    await page.goto('/');
+
+    const response = await responsePromise;
+    expect(response.ok()).toBeTruthy();
+
+    const jsonResponse = await response.json();
+    expect(jsonResponse.FRONTEND_THEME).toStrictEqual('dsfr');
+
+    const footer = page.locator('footer').first();
+    // alt 'Gouvernement Logo' comes from the theme
+    await expect(footer.getByAltText('Gouvernement Logo')).toBeVisible();
+  });
 });