Remove drop shadow - Impress

I’m using a black background on my slideshow, but each slide has a thin white drop shadow, which I am trying to remove.

I’ve gone through the help pages but can’t find any guidance on removing this using a Mac.

Can anyone help me?? Thanks

You might find the discussion on this question useful, especially the workaround suggested by @Grantler, Impress 7.5.2.2, Windows 10, background color does not fill entire slide