Add a dark theme.

lib/main.dart

@@ -25,6 +25,12 @@ class MainApp extends StatelessWidget {
         useMaterial3: true,
         colorScheme: ColorScheme.fromSeed(seedColor: Colors.green),
       ),
+      darkTheme: ThemeData(
+        useMaterial3: true,
+        brightness: Brightness.dark,
+        colorSchemeSeed: Colors.green,
+      ),
+      themeMode: ThemeMode.system,
       home: HomePage(),
     );
   }

lib/pages/expense.dart

@@ -23,9 +23,12 @@ class _ExpensePageState extends State<ExpensePage> {
 
   @override
   Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+
     expenses.sort(
       (a, b) => (b.calcComparableCost() - a.calcComparableCost()).toInt(),
     );
+
     return expenses.isEmpty
         ? Text("Add expenses to get started!")
         : ListView.builder(
@@ -53,7 +56,7 @@ class _ExpensePageState extends State<ExpensePage> {
                     ),
                   ),
                   secondaryBackground: Container(
-                    color: Colors.yellow,
+                    color: Colors.orange,
                     child: Row(
                       mainAxisAlignment: MainAxisAlignment.end,
                       children: [
@@ -90,7 +93,7 @@ class _ExpensePageState extends State<ExpensePage> {
                   child: Container(
                     decoration: BoxDecoration(
                       borderRadius: BorderRadius.circular(4),
-                      color: Colors.greenAccent,
+                      color: theme.colorScheme.onPrimary,
                     ),
                     child: Padding(
                       padding: const EdgeInsets.all(4.0),
@@ -184,9 +187,10 @@ class _ExpenseInputDialogState extends State<ExpenseInputDialog> {
         ),
         AlertDialog(
           insetPadding: EdgeInsets.all(0),
-          title: widget.expense == null
+          title: Center(
-              ? Text("New Expense")
+              child: widget.expense == null
-              : Text("Edit Expense"),
+                  ? Text("New Expense")
+                  : Text("Edit Expense")),
           content: Form(
             key: _expenseFormKey,
             child: Column(