Currently, the customization of ArcGIS Online (particularly Story Maps) is limited by the HTML it supports. A major player missing from the list of supported tags is <link>. This valuable tag would allow the import of:
Users could then keep consistent style between products and create a unique brand away from boiler plate styles--without requiring Enterprise-level solutions. User's style sheets could be uploaded to their AGOL accounts and referenced via URL in the HTML editor of the Story Map.