[enh][m]: update theme (#40)

pages/_app.tsx

@@ -1,11 +1,23 @@
-/* istanbul ignore file */
-import '../styles/index.css';
-
+import { useEffect, useState } from 'react';
 import { ApolloProvider } from '@apollo/react-hooks';
 import { useApollo } from '../lib/apolloClient';
+import { DEFAULT_THEME } from '../themes';
+import { applyTheme } from '../themes/utils';
+
+import './App.css';
 
 export default function MyApp({ Component, pageProps }) {
   const apolloClient = useApollo(pageProps.initialApolloState);
+  const [theme, setTheme] = useState(DEFAULT_THEME);
+
+  useEffect(() => {
+    /**
+     * We can switch theme.
+     * e.g. setTheme('primary');
+     * */
+
+    applyTheme(theme);
+  }, [theme]);
 
   return (
     <ApolloProvider client={apolloClient}>