fix(playground): fix css dark mode

Author: fu050409

playground/src/Activate.tsx

@@ -1,7 +1,7 @@
-import { useState } from "react";
 import { Button } from "@matechat/react/button";
 import { createOpenAIBackend } from "@matechat/react/utils/backend";
 import { agent } from "@matechat/react/utils/core";
+import { useState } from "react";
 
 export function Activate({ onActivate }: { onActivate: () => void }) {
   const [token, setToken] = useState("");

playground/src/style.css

@@ -1,2 +1,3 @@
 @import "tailwindcss";
 @source "../../src/";
+@custom-variant dark (&:where(.dark, .dark *));