CSS: Styling expanded nodes only?

[Lukas]

I am by far no expert, but I think this is not possible with pure CSS.

You first would need to identify the .Node-self.is-parent:not(.is-collapsed) elements (no problem until here), but then modify the parent .Node or grandparent .Node-outer element, which is not possible according to

@Piotr

Unfortunately updating parents styles when something is done to children* is not possible in pure CSS.

But I would be delighted if someone could proof me wrong!

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P.S.: If you are fine with only highlighting the child items, this might give you a pointer:

.Node:not(.is-currentRoot) > .Node-self.is-parent:not(.is-collapsed) + .Node-children
{
    Background: rgba(0,0,0,.1);
}