center back button extracting the widget

lib/src/thread_header.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:stream_chat/stream_chat.dart';
+import 'package:stream_chat_flutter/src/back_button.dart';
 import 'package:stream_chat_flutter/src/stream_chat_theme.dart';
 
 /// ![screenshot](https://raw.githubusercontent.com/GetStream/stream-chat-flutter/master/screenshots/thread_header.png)
@@ -80,7 +81,12 @@ class ThreadHeader extends StatelessWidget implements PreferredSizeWidget {
           StreamChatTheme.of(context).channelTheme.channelHeaderTheme.color,
       actions: <Widget>[
         Container(
-          child: showBackButton ? _buildBackButton(context) : Container(),
+          child: showBackButton
+              ? StreamBackButton(
+                  onPressed: onBackPressed,
+                  icon: Icons.close,
+                )
+              : SizedBox(),
         ),
       ],
       centerTitle: false,
@@ -104,38 +110,6 @@ class ThreadHeader extends StatelessWidget implements PreferredSizeWidget {
     );
   }
 
-  Padding _buildBackButton(BuildContext context) {
-    return Padding(
-      padding: const EdgeInsets.all(14.0),
-      child: AspectRatio(
-        aspectRatio: 1,
-        child: RawMaterialButton(
-          shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(4)),
-          elevation: 0,
-          highlightElevation: 0,
-          focusElevation: 0,
-          disabledElevation: 0,
-          hoverElevation: 0,
-          onPressed: () {
-            if (onBackPressed != null) {
-              onBackPressed();
-            } else {
-              Navigator.of(context).pop();
-            }
-          },
-          fillColor: Theme.of(context).brightness == Brightness.dark
-              ? Colors.white.withOpacity(.1)
-              : Colors.black.withOpacity(.1),
-          padding: EdgeInsets.all(4),
-          child: Icon(
-            Icons.close,
-            size: 15,
-          ),
-        ),
-      ),
-    );
-  }
-
   @override
   final Size preferredSize;
 }