chore(preconstruct): inline css via babel config, add themes package

examples/nextjs/pages/_app.js

@@ -0,0 +1,7 @@
+import '../styles/globals.css'
+
+function MyApp({ Component, pageProps }) {
+  return <Component {...pageProps} />
+}
+
+export default MyApp

examples/nextjs/pages/index.js

@@ -0,0 +1,10 @@
+import ReactFlow from '@react-flow/core';
+import '@react-flow/themes/dark';
+
+export default function Home() {
+  return (
+    <div style={{ height: '100vh' }}>
+      <ReactFlow defaultNodes={[]} defaultEdges={[]} />
+    </div>
+  );
+}