add default user image to theme

example/lib/main.dart

@@ -92,13 +92,6 @@ class MyApp extends StatelessWidget {
       themeMode: ThemeMode.system,
       home: Container(
         child: StreamChat(
-          streamChatThemeData: StreamChatThemeData(
-            defaultChannelImage: (context, channel) {
-              return Center(
-                child: Text('AAA'),
-              );
-            },
-          ),
           client: client,
           child: ChannelListPage(),
         ),

lib/src/stream_chat.dart

@@ -113,6 +113,7 @@ class StreamChatState extends State<StreamChat> with WidgetsBindingObserver {
     final theme = defaultTheme.copyWith(
       primaryColor: themeData?.primaryColor,
       defaultChannelImage: themeData?.defaultChannelImage,
+      defaultUserImage: themeData?.defaultUserImage,
       channelTheme: defaultTheme.channelTheme.copyWith(
         channelHeaderTheme:
             defaultTheme.channelTheme.channelHeaderTheme.copyWith(

lib/src/stream_chat_theme.dart

@@ -54,6 +54,9 @@ class StreamChatThemeData {
   /// The widget that will be built when the channel image is unavailable
   final Widget Function(BuildContext, Channel) defaultChannelImage;
 
+  /// The widget that will be built when the user image is unavailable
+  final Widget Function(BuildContext, User) defaultUserImage;
+
   /// Create a theme from scratch
   StreamChatThemeData({
     this.primaryColor,
@@ -64,6 +67,7 @@ class StreamChatThemeData {
     this.otherMessageTheme,
     this.ownMessageTheme,
     this.defaultChannelImage,
+    this.defaultUserImage,
   });
 
   /// Create a theme from a Material [Theme]
@@ -103,12 +107,14 @@ class StreamChatThemeData {
     MessageTheme ownMessageTheme,
     MessageTheme otherMessageTheme,
     Widget Function(BuildContext, Channel) defaultChannelImage,
+    Widget Function(BuildContext, User) defaultUserImage,
   }) =>
       StreamChatThemeData(
         primaryColor: primaryColor ?? this.primaryColor,
         secondaryColor: secondaryColor ?? this.secondaryColor,
         accentColor: accentColor ?? this.accentColor,
         defaultChannelImage: defaultChannelImage ?? this.defaultChannelImage,
+        defaultUserImage: defaultUserImage ?? this.defaultUserImage,
         channelPreviewTheme: channelPreviewTheme?.copyWith(
               title:
                   channelPreviewTheme.title ?? this.channelPreviewTheme.title,
@@ -178,6 +184,12 @@ class StreamChatThemeData {
       accentColor: accentColor,
       primaryColor: isDark ? Colors.black : Colors.white,
       defaultChannelImage: (context, channel) => SizedBox(),
+      defaultUserImage: (context, user) => Center(
+        child: Text(
+          user.name?.substring(0, 1) ?? '',
+          style: TextStyle(color: Colors.white),
+        ),
+      ),
       channelPreviewTheme: ChannelPreviewTheme(
         avatarTheme: AvatarTheme(
           borderRadius: BorderRadius.circular(20),

lib/src/user_avatar.dart

@@ -43,7 +43,7 @@ class UserAvatar extends StatelessWidget {
                 },
                 fit: BoxFit.cover,
               )
-            : SizedBox(),
+            : StreamChatTheme.of(context).defaultUserImage(context, user),
       ),
     );
   }