feat: improve app theming code and add Material You (#58)

lib/ui/widgets/shared/custom_card.dart

@@ -0,0 +1,32 @@
+import 'package:flutter/material.dart';
+
+class CustomCard extends StatelessWidget {
+  final bool isFilled;
+  final Widget child;
+
+  const CustomCard({
+    Key? key,
+    this.isFilled = true,
+    required this.child,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: BoxDecoration(
+        borderRadius: BorderRadius.circular(16),
+        color: isFilled
+            ? Theme.of(context).colorScheme.secondaryContainer
+            : Colors.transparent,
+        border: isFilled
+            ? null
+            : Border.all(
+                width: 1,
+                color: Theme.of(context).colorScheme.secondary,
+              ),
+      ),
+      padding: const EdgeInsets.all(20),
+      child: child,
+    );
+  }
+}