Add Manifold theme to Storybook

[drwpow]

Reason for change

When we’re developing styles, we weren’t looking at our own styles! Let’s use our styles at least as the default.

This also has a secondary benefit of providing our design team with accurate, interactive versions of our component system.

Testing

Check out the new Storybook preview link below under Now (Heroku is still used to show the Gatsby docs)! 🎉

[vercel]

This pull request is automatically deployed with Now.
To access deployments, click Details below or on the icon next to each push.

Latest deployment for this branch: https://ui-git-dangodev-manifold-styles.manifold.now.sh

[codecov]

Codecov Report

Merging #264 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #264   +/-   ##
=======================================
  Coverage   60.11%   60.11%           
=======================================
  Files          42       42           
  Lines        1008     1008           
  Branches      235      235           
=======================================
  Hits          606      606           
  Misses        387      387           
  Partials       15       15

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[markjamieson]

This fixes an issue I saw when reviewing some @Minivera PRs. It had seemed like Storybook wasn't picking up on some of the global vars, but this seems to fix it!