systemNavigationBarColor ← appBar.backgroundColor

Makes the system navigation bar color (on Android) follow the color of
the appBar (and hence of the spaces bar).

See also https://gitlab.com/famedly/fluffychat/-/issues/699.

lib/main.dart

@@ -146,7 +146,8 @@ class _FluffyChatAppState extends State<FluffyChatApp> {
                 SystemChrome.setSystemUIOverlayStyle(
                   SystemUiOverlayStyle(
                     statusBarColor: Colors.transparent,
-                    systemNavigationBarColor: Theme.of(context).backgroundColor,
+                    systemNavigationBarColor:
+                        Theme.of(context).appBarTheme.backgroundColor,
                     systemNavigationBarIconBrightness:
                         Theme.of(context).brightness == Brightness.light
                             ? Brightness.dark