Error in the display of styles iframe

Hi.

Thanks for the website. Can add the fix. And that breaks the display.



```styl
#iframe
  background-color: #fff
```



https://github.com/vuejs-tips/vuex-cheatsheet/blob/debf49e84945580d299a10c78a38f31310e22c8e/src/styles/modal.styl#L31


![vuex-error](https://user-images.githubusercontent.com/1606172/47563443-80f20a80-d92a-11e8-892e-04068acd99f3.jpg)


