Fix Storybook styles (#8133)

When the storybook config was migrated to the new module structure in
PR #8112 I forgot to include the line where the styles were imported.

.storybook/preview.js

@@ -1,6 +1,7 @@
 import React from 'react'
 import { addDecorator, addParameters } from '@storybook/react'
 import { withKnobs } from '@storybook/addon-knobs/react'
+import '../ui/app/css/index.scss'
 
 addParameters({
   backgrounds: [