Update gatsby to 4.8.1

• Update gatsby and packages to resolve dependency error
• Fix theme to work with new version
• Fix configs

src/@rocketseat/gatsby-theme-docs/styles/global.js

@@ -2,9 +2,25 @@ import React from 'react';
 import { Global, css } from '@emotion/core';
 import { useTheme } from 'emotion-theming';
 import { lighten } from 'polished';
+import { theme } from './theme';
 
 export default function GlobalStyle() {
-  const theme = useTheme();
+  const theme = {
+    colors: {
+      primary: '#E1A809',
+      background: '#fff',
+      sidebar: {
+        background: '#ffffff',
+        link: '#737373',
+        heading: '#aaa',
+        linkActive: '#13131A',
+        itemActive: '#F5F5FA',
+        footer: '#A8A8B3',
+      }
+    }
+  };
+
+  console.log(theme);
 
   return (
     <Global