fixes (#17624)

packages/flutter/lib/src/material/app_bar.dart

@@ -345,6 +345,7 @@ class _AppBarState extends State<AppBar> {
     TextStyle centerStyle = widget.textTheme?.title ?? themeData.primaryTextTheme.title;
     TextStyle sideStyle = widget.textTheme?.body1 ?? themeData.primaryTextTheme.body1;
 
+    if (parentRoute?.isCurrent ?? true) {
       final Brightness brightness = widget.brightness ?? themeData.primaryColorBrightness;
       // TODO(jonahwilliams): remove once we have platform themes.
       switch (defaultTargetPlatform) {
@@ -357,6 +358,7 @@ class _AppBarState extends State<AppBar> {
         case TargetPlatform.fuchsia:
           SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle.dark);
       }
+    }
 
     if (widget.toolbarOpacity != 1.0) {
       final double opacity = const Interval(0.25, 1.0, curve: Curves.fastOutSlowIn).transform(widget.toolbarOpacity);