chatview: make data transfer send container scrollbar handle visible

Change-Id: Icdbf47c76ab5725ccb855634252f7a1a9f9228ce

src/web-chatview/chatview.css

@@ -369,7 +369,15 @@ a:hover {
 }
 
 #data_transfer_send_container::-webkit-scrollbar {
-    height: 10px;
+    height: 8px;
+}
+
+#data_transfer_send_container::-webkit-scrollbar-track {
+    background-color: var(--message-out-bg);
+}
+
+#data_transfer_send_container::-webkit-scrollbar-thumb {
+    background-color: var(--bg-color);
  }
 
 #data_transfer_send_container::after {