Using Emotion seems to break MDX

[rsmidt]

Describe the bug
Using Emotion seems to break stories in MDX format. The following stack trace is from the auto generated Introduction.stories.mdx:

Uncaught (in promise) TypeError: theme.background is undefined
    DocsWrapper DocsPage.js:45
    handleInterpolation emotion-serialize.browser.esm.js:137
    serializeStyles2 emotion-serialize.browser.esm.js:251
    Styled emotion-styled-base.browser.esm.js:110
    withEmotionCache2 emotion-element-a8309070.browser.esm.js:27
    React 12
    unstable_runWithPriority scheduler.development.js:468
    React 7
    renderDocs StoryRenderer.js:462
    _callee2$ StoryRenderer.js:258
    Babel 8
    _callee$ StoryRenderer.js:160
    Babel 8
    forceReRender StoryRenderer.js:103
    node_modules chunk-7D4YIRY2.js:1336
    node_modules chunk-FLULKQ2P.js:632
    handleEvent index.js:167
    handler2 index.js:93
    emit index.js:100
    updateGlobals story_store.js:826
    node_modules chunk-FLULKQ2P.js:2899
    node_modules chunk-FLULKQ2P.js:632
    handleEvent index.js:167
    node_modules chunk-FLULKQ2P.js:526
    node_modules chunk-7D4YIRY2.js:276
    handleEvent index.js:258
DocsPage.js:45:5

To Reproduce
The repo including the storybook can be found here: https://github.com/vocabularee/web/tree/voc-38-add-storybook-for-building-ui-components.

System

Environment Info:

  System:
    OS: Linux 5.12 Manjaro Linux
    CPU: (8) x64 Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz
  Binaries:
    Node: 14.17.1 - ~/.nvm/versions/node/v14.17.1/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v14.17.1/bin/yarn
    npm: 6.14.13 - ~/.nvm/versions/node/v14.17.1/bin/npm
  Browsers:
    Firefox: 89.0.2
  npmPackages:
    @storybook/addon-actions: ^6.4.0-alpha.3 => 6.4.0-alpha.3
    @storybook/addon-essentials: ^6.4.0-alpha.3 => 6.4.0-alpha.3
    @storybook/addon-links: ^6.4.0-alpha.3 => 6.4.0-alpha.3
    @storybook/react: ^6.4.0-alpha.3 => 6.4.0-alpha.3

Additional context
I'm using the experimental? storybook-builder-vite.

[ndelangen]

This is fixed in storybook 7.0 beta.