Here are some mock-ups about how the make the top-level categories look different from the sub-categories.
The last one is the one that took the most time. In that one, the top category is pretty normal, but the sub-categories each have a new icon. So for example, “Common conditions for all objects” doesn’t have an icon, but “Position”, “Angle”, “Collisions” etc have their own icons.
It would also be possible to do it the other way round - showing an icon for the top-level categories like “Common conditions for all objects”, “Text objects”, “Scenes” and so on, but not for the lower-level categories. I ended up spending quite a bit of time on this, so I didn’t end up doing that.
Why did this take so much time, then? Because I realized all the icons for all the properties need to be reworked, too!