create MainScreen.js

+import React from "react";
+import styled from "styled-components";
+import ChatHeader from "./ChatHeader";
+import ChatScreen from "./ChatScreen";
+import ChatFooter from "./ChatFooter";
+
+const MainScreen = styled.div`
+  display: flex;
+  flex-direction: column;
+`;
+
+export default () => {
+  return (
+    <MainScreen>
+      <ChatHeader />
+      <ChatScreen />
+      <ChatFooter />
+    </MainScreen>
+  );
+};