NOTE: Test it with default theme
# Create empty file themes/default/inc/styles.js
# Login to admin
# Go to {nav Configuration > Website > Themes}
# Open default theme for editing
# Put `inc/styles.css` into `Stylesheet File` and click `Save`
# Go to {nav Website & Content > Browse Website}, switch to `Content Mode` tab
# Click on `Edit Content` button
# Confirm content in editor loads with background