Fix SimplePage background color variable (#232)

* Use platform-specific canvas color for SimplePage

* Use platform-specific icon for MainScreen appbar

lib/components/SimplePage.dart

@@ -1,4 +1,3 @@
-import 'package:flutter/cupertino.dart' as cupertino;
 import 'package:flutter/material.dart';
 import 'package:flutter_platform_widgets/flutter_platform_widgets.dart';
 import 'package:mobile_nebula/services/utils.dart';
@@ -99,7 +98,7 @@ class SimplePage extends StatelessWidget {
     }
 
     return PlatformScaffold(
-        backgroundColor: cupertino.CupertinoColors.systemGroupedBackground.resolveFrom(context),
+        backgroundColor: Theme.of(context).scaffoldBackgroundColor,
         appBar: PlatformAppBar(
           title: title,
           leading: leadingAction != null ? leadingAction : Utils.leadingBackWidget(context),

lib/screens/MainScreen.dart

@@ -157,7 +157,7 @@ class _MainScreenState extends State<MainScreen> {
       trailingActions: <Widget>[
         PlatformIconButton(
           padding: EdgeInsets.zero,
-          icon: Icon(Icons.menu, size: 28.0),
+          icon: Icon(Icons.adaptive.more, size: 28.0),
           onPressed: () => Utils.openPage(context, (_) => SettingsScreen(widget.dnEnrollStream)),
         ),
       ],