I have a very simple UI. A MarginContainer that contains a VBoxContainer which has a couple of labels. Now I want to give the MarginContainer a slightly transparent dark grey background but I don't see how.

I already have a theme for the top DebugUI control and can modify attributes of the child controls just fine. I was able to change the background color of those labels but that isn't what I want because the MarginContainer defines a small margin and I want it to change its background color as well.

Do I need to add a shader or something? I feel like I am missing something fundamentally here.

Okay I just found one solution and that was embedding the MarginContainer into a PanelContainer and then changing its StyleBoxFlat panel theme attribute. This works but... there has to be a simpler way, right?