CSS broken in docs-staging

[jennifer-shehane]

This commit seems to have broken the css in the https://docs-staging.cypress.io page. 5be5de4

See the Cypress Test Video of the run, where the css is obviously not loading correctly: https://dashboard.cypress.io/#/projects/ma3dkn/runs/c28f9d36-1eef-4bcf-b4fb-8e9a4732ac4f/videos

And the Cypress Test Video on the commit before, where the css is fine: https://dashboard.cypress.io/#/projects/ma3dkn/runs/480835eb-3b6b-490d-a7c7-259a1275820c/videos

We need to:

1. Fix the bug
2. Write a test that ensures css is compiling and loading correctly after the deployment job on CI.

Can recreate locally running NODE_ENV=staging npm start. Ensure you are on Node 8 and have run npm i first.

[bahmutov]

When style.css is bundled correctly it will have rules for things like `#container', which makes it height 100% of the viewport

we can do two tests:

• make sure the container height is equal to viewport height
• loaded document CSS rules have rules for #container element