Hey as much of a beginner as you but I have been able to get done what you were doing. You have to add open the BACKDROP attribute up and and add <AnchorFill />. Like this.
Code:
<Backdrop name="$(parent)BG" centerColor="adadad" alpha="0.8">
<AnchorFill />
<Edge edgeSize="0" />
</Backdrop>
Igonore the edgeSize for some reason I am getting a background with the adadad color but it has a 5 or so pixel edge around it.