add stream chat theme draft

lib/src/user_avatar.dart

@@ -2,6 +2,8 @@ import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 import 'package:stream_chat/stream_chat.dart';
 
+import '../stream_chat_flutter.dart';
+
 class UserAvatar extends StatelessWidget {
   const UserAvatar({
     Key key,
@@ -14,16 +16,28 @@ class UserAvatar extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return CircleAvatar(
-      radius: radius,
-      backgroundImage: user.extraData.containsKey('image')
-          ? CachedNetworkImageProvider(user.extraData['image'] as String)
-          : null,
-      child: user.extraData.containsKey('image')
-          ? null
-          : Text(user?.extraData?.containsKey('name') ?? false
-              ? user.extraData['name'][0]
-              : ''),
+    return Container(
+      padding: const EdgeInsets.all(4),
+      constraints:
+          StreamChatTheme.of(context).messageTheme.avatarTheme.constraints,
+      decoration: BoxDecoration(
+        borderRadius:
+            StreamChatTheme.of(context).messageTheme.avatarTheme.borderRadius,
+        color: StreamChatTheme.of(context).accentColor,
+      ),
+      child: user.extraData?.containsKey('image') ?? false
+          ? CachedNetworkImage(
+              imageUrl: user.extraData['image'],
+              errorWidget: (_, __, ___) {
+                return Center(
+                  child: Text(user.extraData?.containsKey('name') ?? false
+                      ? user.extraData['name'][0]
+                      : ''),
+                );
+              },
+              fit: BoxFit.cover,
+            )
+          : SizedBox(),
     );
   }
 }