Following Configure general settings—ArcGIS Online Help | Documentation I set up and save an organizational theme:
etc etc...
That same page notes:
Specify the brand colors and logo (if needed) that you want to apply to Web AppBuilder apps, supported ArcGIS Configurable Apps templates, story maps created with classic Esri Story Maps, and Open Data sites created by members of your organization. To specify colors, click Manage shared theme colors and set the colors for your headers, buttons, and body. A contrast ratio appears for the shared theme colors you specify. Contrast ratio is a measure of legibility based on WCAG 2.1 accessibility standards; a contrast ratio of 4.5 is recommended to adhere to these standards. and ...Currently, most classic Story Maps and other configurable apps support Shared theme.
I'm not sure how this theme works, or how the user accesses it when creating an app, in particular the latest issue of story maps...
Ian Wittenmyer
Owen Evans
Kelly Gerrow
That should just about do it....