I would like to know where the python code/stylesheet/.qss etc for the native networkEditor>QTreeView(Operator List) are located.
For purposes of setting various QT settings, e.g

TreeView.setSelectionMode(multi selection is sorely missing)
TreeView.customContextMenuRequested.connect()
TreeView.setIconSize()
QTreeView::branch:has-siblings:!adjoins-item {border-image: none;}
* And much more!

LEFT: Custom QTreeView, much cleaner for my eyes.
RIGHT: Native houdini treeview, would like to tweak, thanks!

AFAIK this is not implemented in Qt. The only way to do what you want is to create your own from scratch as a Python panel.