Do you mean something like what Gramps Web describes in its Features : Powerful map" ?
Or the new 5.2 Feature that for Custom Tile Providers (which needs a volunteer to explore and write wiki docs in the “How to change the map provider ?” section of Geography)
See:
- MantisBT request 11416: Geography View: Add custom tile providers
- Merged Pull Request 976: Geography: Add custom tiles provider
- Discourse thread: “Feature preview: hack custom map provider tiles”