This confused me at first, so to be clear, this refers to the expand/collapse arrows beside layers that have sub-layers. Here's how they appear in the default theme:

The design of the dark/light themes however intentionally adjusts the appearance of the layers bar and hides these buttons, as the theme design pre-dates sub-layers. Simply showing the buttons again looks odd, so the design will probably have to be revised to suit them. I think the dark/light themes should be reverted to the standard arrangement of the Layers Bar.

I want to do a follow up on this as this is something that is quite important imo, for the following reason:
Without the cascade arrow it is impossible to tell that a collapsed layer has any sub-layers.
So this could potentially make it almost impossible to find stuff because the user might not even know that there is a sub-layer somewhere. If the person realizes they would have to double click every layer to see if there are sub-layers.

Any updates on adding this to the dark UI?

I've just reverted the Layers Bar to the same layout as the default theme in both the dark and light themes for the next release cycle (the change will be in the first beta release after the next stable release).