feat(ui): add dark theme

lib/main.dart

@@ -27,8 +27,14 @@ class MyApp extends StatelessWidget {
     return MaterialApp(
       title: 'Oeffimaster',
       theme: ThemeData(
-        colorScheme: .fromSeed(seedColor: Colors.yellow),
+        brightness: Brightness.light,
+        colorSchemeSeed: Colors.yellow,
       ),
+      darkTheme: ThemeData(
+        brightness: Brightness.dark,
+        colorSchemeSeed: Colors.yellow,
+      ),
+      themeMode: ThemeMode.system,
       // home: const MyHomePage(title: 'Oeffimaster'), //this is what matters
       home: const JourneyOverviewPage(),
       // routes: {
@@ -108,6 +114,8 @@ class _MyHomePageState extends State<MyHomePage> {
         ),
       ),
       floatingActionButton: FloatingActionButton(
+        backgroundColor: Theme.of(context).colorScheme.primaryContainer,
+        foregroundColor: Theme.of(context).colorScheme.onPrimaryContainer,
         onPressed: () {
           Navigator.push(
             context,
@@ -366,6 +374,8 @@ class _ChooseDeparturePageState extends State<ChooseDeparturePage> {
         ),
       ),
       floatingActionButton: FloatingActionButton(
+        backgroundColor: Theme.of(context).colorScheme.primaryContainer,
+        foregroundColor: Theme.of(context).colorScheme.onPrimaryContainer,
         onPressed: () {
           if (widget.journeyId != null) {
             loadJourney(widget.journeyId as int);
@@ -770,6 +780,8 @@ class _JourneyOverviewPageState extends State<JourneyOverviewPage> {
         ),
       ),
       floatingActionButton: FloatingActionButton(
+        backgroundColor: Theme.of(context).colorScheme.primaryContainer,
+        foregroundColor: Theme.of(context).colorScheme.onPrimaryContainer,
         onPressed: () {
           Navigator.push(
             context,