week_03: Codepen background color change issue

Related to:
https://github.com/shift-up/coursebook/blob/master/course/week_03.md

There is an issue that seems to be due to CodePen where changing the body background color doesnt work:
https://codepen.io/nballaney/pen/oNNNmbJ

It doesnt seem to happen on Chrome or FF, but likely on IE.