please add a style for all containers
currently things like hbox and vbox cannot render anything forcing the user to place something behind it

No, I don't think it makes sense. Containers are for rearranging their children, they do not need a style.
If you need a style to be applied, simply put your container inside a PanelContainer.
I don't want to outright reject the idea, but I don't see what you might want to style about the containers. So I'll ask: If we were to allow styling these containers, what would you use that styling for? What might you expect to be able to style about a container?
Having styling on containers would allow for not using PanelContainer so often, which would lead to somewhat cleaner hierarchies.
Having styling on containers would allow for not using PanelContainer so often, which would lead to somewhat cleaner hierarchies.
The thing is that I am not sure it makes sense for most containers. It has also a maintenance cost, unless we add it as a generic property in all containers. Also, this would make the PanelContainer useless.
All of this could be modified, but it should be a global rework, not just adding style to all containers.
Containers are all CanvasItem still, which means they always create a CanvasItem RID and they can draw with it. For now you could use a script which draws your stylebox in _draw().
the purpose of styling these would be so that the user doesn't have to place a panel underneath the container.
if containers shouldn't be stylized then there should be some sort of panel alternative to all the containers in which can be styled.
This doesn't seem like something that should be implemented in Godot. Since there are a number of ways to get this effect with what Godot provides, this would just add bloat.
can't this be done with a theme?
@girng no it can't that's the issue
I mean... you can pick the StyleBox you want and assign it to all container items in a custom Theme resource, and define that as your global theme. That will make all of these container have that stylebox by default, a bit like in CSS. Did you try that and it failed?
Edit: indeed, you're right...

So the only alternative for you would be to create a scene with a container having the background you want (either by child Panel or script) and use instances of that instead of creating containers from the Add Node menu.
@Zylann wait what?
your example image doesn't have a option to assign a StyleBox.
@Shadowblitz16 yeah I just realized.
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No, I don't think it makes sense. Containers are for rearranging their children, they do not need a style.
If you need a style to be applied, simply put your container inside a PanelContainer.