In section “Create Clear Boundaries With Subgraphs” the subgraph feature is introduced as a boundary. On page 57 within list item 2 it is described how the style feature is used to remove the “virbrant yellow” background of the subgraph.
But it is ommited, that subgraphs usually have a title, that is now hidden via setting the title’s color to white. Just a tiny but helpful detail .