How do I make the @ContentBackground transparent

I can change the color via the Apperance/Styles/General/Content/ here I can change the background color or even add an image. However I cant make it transparent, it defaults back to the native color.

Can I change the hard code of this in the Apperance/templates? if so, what file do I need edit and where in that file is the code?
tried that still will just revert back to a default color :(
thats why I think I need to go into the hardcode itself to bypass this easy custom changing.
it keeps defaulting to #181818.. so I exported my theme.xml file and searched within it for the color code #181818 and it is not in there... so it must be getting this from some other source :(