Use storybook-dark-mode addon to toggle light/dark mode

See the project https://github.com/hipstersmoothie/storybook-dark-mode

packages/storybook/.storybook/main.js

@@ -9,6 +9,7 @@ module.exports = {
     '@storybook/addon-actions',
     '@storybook/addon-links',
     '@storybook/addon-storysource',
+    'storybook-dark-mode/register',
   ],
   webpackFinal: async config => {
     config.resolve.alias = {

packages/storybook/package.json

@@ -18,7 +18,8 @@
     "@storybook/addon-links": "^5.3.17",
     "@storybook/addon-storysource": "^5.3.18",
     "@storybook/addons": "^5.3.17",
-    "@storybook/react": "^5.3.17"
+    "@storybook/react": "^5.3.17",
+    "storybook-dark-mode": "^0.4.1"
   },
   "dependencies": {
     "@backstage/theme": "0.1.1-alpha.4"