Uneditable grey background when rounding corners for Lightbox?

Hey! Trying to round off the corners for the lightbox window (hard angles are for squares, lol) and am encountering an issue where there is a light grey background that peeks out from behind when you increase the corner radius in the properties panel. I’ve hidden the superfluous horizontal scrollbar that squares out the bottom corners (is that a bug or something?) but am not sure what ID or class I’d use for styling. Anyone know how to make that lightbox background transparent (or accomplish my goal in another way) ?

I think this was asked once before in like 2016, but there were no answers and it’s on an old forum so I thought I’d re-ask. Apologies if that’s not the right move, but thanks for any help!

Edit: also, completely separate, but can i use a custom button to close out the lightbox ? I can’t find a native option and I’m not a fan of having to use the built in ‘X’.