Is it a problem to load multiple CSS? Not sure to see the issue here.
So to my knowledge there’s already some discussions for dropping jstree in the future, @CharpentierLucas opened a thread for discussing that here: Removing the jstree dependency: overhaul of the tree macro
Now it’s probably a long run decision and we certainly need a short term solution for FA.
Maybe we can find a workaround by replacing those icons dynamically with a piece of JS once the tree is loaded?