added dark mode for reference

www/reference/package.json

@@ -18,10 +18,12 @@
     "@emotion/react": "^11.1.2",
     "@emotion/styled": "^11.0.0",
     "@rebass/forms": "^4.0.6",
+    "copy-to-clipboard": "^3.3.1",
     "emotion-theming": "^10.0.27",
     "gatsby": "^3.9.1",
     "gatsby-plugin-algolia-docsearch": "^1.0.5",
     "gatsby-plugin-anchor-links": "^1.1.1",
+    "gatsby-plugin-dark-mode": "^1.1.2",
     "gatsby-plugin-emotion": "^5.0.0",
     "gatsby-plugin-env-variables": "^2.1.0",
     "gatsby-plugin-preact": "^5.9.0",
@@ -38,14 +40,13 @@
     "preact-render-to-string": "^5.1.19",
     "prismjs": "^1.24.1",
     "react": "^16.12.0",
-    "copy-to-clipboard": "^3.3.1",
     "react-collapsible": "^2.8.1",
     "react-dom": "^16.12.0",
     "react-helmet": "^6.1.0",
     "react-highlight.js": "^1.0.7",
-    "react-tooltip": "^4.2.10",
     "react-intersection-observer": "^8.29.0",
     "react-markdown": "^5.0.3",
+    "react-tooltip": "^4.2.10",
     "react-virtualized": "^9.22.3",
     "theme-ui": "^0.10.0"
   },