Add CSS through the Admin UI

CSS can be added on a page by page basis through the Admin UI. To do so, visit a content edit page in the Admin, such as the home page edit screen.

Hover over the Content card and click the "Manage Content" button.

Click the "Advanced" link at the top of the Content Editor UI.

Type CSS into the "CSS" field and save.

Visit the home page in the Storefront and behold! CSS!