I don't think you can change Curtain transparency via Start10 setting. You will need Curtain style that have transparency in it. That is why I suggest my style above since I know that one have transparency in Start Menu, Taskbar and Windows frames. And I wanted to see if it appear on your system.
If a style do not have transparency in it, you will not have transparent part in it.
Thank you,
Basj,
Stardock Community Assistant