I read in the forum that the “structural” CSS in Joplin is not exposed, so I don’t believe I can do this in external CSS.
One thing that would be very nice would be to have the option for each child notebook tab to have a background color that is, say, 10% lighter than its parent.
That would make the data structure much more visible than just indentation, which is useful when you’re trying to “scan down” into categories.