modify checkbox to have 3 radius

lib/src/message_input.dart

@@ -235,6 +235,16 @@ class MessageInputState extends State<MessageInput> {
             child: Container(
               height: 16,
               width: 16,
+              foregroundDecoration: BoxDecoration(
+                border: _sendAsDm
+                    ? null
+                    : Border.all(
+                        color: Colors.black.withOpacity(.5),
+                        width: 2,
+                      ),
+                borderRadius: BorderRadius.circular(3),
+              ),
+              child: Center(
                 child: Material(
                   borderRadius: BorderRadius.circular(3),
                   color: _sendAsDm
@@ -253,7 +263,7 @@ class MessageInputState extends State<MessageInput> {
                           ? CrossFadeState.showFirst
                           : CrossFadeState.showSecond,
                       firstChild: Icon(
-                      Icons.check,
+                        StreamIcons.check,
                         size: 16.0,
                         color: Colors.white,
                       ),
@@ -266,6 +276,7 @@ class MessageInputState extends State<MessageInput> {
                 ),
               ),
             ),
+          ),
           Padding(
             padding: const EdgeInsets.symmetric(horizontal: 8.0),
             child: Text('Send also as direct message'),