Owen Evans writes:
A shared theme is a new feature of the March 2017 release of ArcGIS Online that allows administrators to configure a logo and color palette for consistent branding of configurable apps created by members of their organization.
Read the blog post to see how you (well, your Organization's admin actually) can define your organization's color scheme and logo once and have them appear in the story maps that your team creates...
For example, here's the new option to use the logo for the organization that has been defined as part of the shared theme. If a logo has been defined there, you'll see this option: