feat: New chat background colors and chatlist design

lib/widgets/background_gradient_box.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+
+class BackgroundGradientBox extends StatelessWidget {
+  final Widget child;
+  const BackgroundGradientBox({
+    Key key,
+    this.child,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      decoration: BoxDecoration(
+        gradient: LinearGradient(
+          begin: Alignment.topRight,
+          end: Alignment.bottomLeft,
+          colors: [
+            Theme.of(context).scaffoldBackgroundColor,
+            Theme.of(context).secondaryHeaderColor,
+          ],
+        ),
+      ),
+      child: child,
+    );
+  }
+}