Thanks for the answer @rgawenda.
Using the above code and adding
I still don't see my image in the tree-view. I have also tried
url("styles/img.png"), which still does not work.
Can I confirm that
tree-view-background is a default element in Atom, and hasn't been added by the tree-view-background package? I would like to be able to export this theme for use without requiring the tree-view-background package.