Ok, i just checked your video. This didn't help in my case. I use an OptionButton. That widget is done by Godot. I cannot access the internal PopupMenu via editor, only via code. Or i don't know how to do it. So yes, i can add different children types to this widget, but i am not able to style them. How can i set a theme for a Label i add to the OptionButton.
$OptionButton.add_Item("item no 1")
$OptionButton.get_popup().add_item("item no 2")
# change theme or font of thise elements here
So i am able to add children to the OptionButton, but i cannot change the style of those children.