How to use flexbox with ember?

I am struggling to use flexbox with ember. I installed the addon ember-flexboxgrid-sass

It says to add @import flexboxgrid; to the app.scss file. But I didn't have an app.scss file. So I created one.

Then I thought maybe I needed a sass addon so I installed ember-cli-sass.

That didn't work either. I feel like it should be straight forward but I can't get it working.

Does anyone have any suggestions?