Example: override App component with extra header on top

_override/App.tsx

@@ -0,0 +1,13 @@
+import * as React from 'react';
+import type { Location } from 'history';
+
+export function App({ children, location }: React.PropsWithChildren<{ location: Location }>) {
+  return (
+    <>
+      <header style={{background: 'yellow', textAlign: 'center', padding: '20px 0', fontSize: '24px'}}>
+        Header
+      </header>
+      {children}
+    </>
+  );
+}