[Question] How to change folder icon for "open" state?

[despecial]

Using the latest TB version (78), I am trying to change the tree icon when the folder is open and has content.
So far I was able to change all folders but not these. Any hints?

[Aris-t2]

I could only figure out how to change same icons in their default state.

Files like foldericons_tb1.css contain what is possible atm.

[despecial]

Thanks for getting back. I would love to add an "open folder" icon when the arrow is pointing downwards