Note that cms.js will be used in two different places, in application.js (used by Scrivito's details views) and in frontend/all.js (used by the Spree/Solidus layout).
Create a new file, app/assets/stylesheets/cms.css, and set its contents to:
*= require scrivito
*= require scrivito with
*= require cms.
*= require cms to vendor/assets/stylesheets/spree/frontend/all.css
The app/assets/stylesheets/cms.css file is very small, too, but this is where all styles related to the Scrivito CMS should be placed.