equivalent to "slide background fill" in impress

I try to recreate this pp tutorial in libreoffice impress…

I can with anything but the step shown in the 7:30 min.
Can someone help me?

Thanks in advance!

It looks to me like this feature isn’t (yet?) available in Impress. I tried mimicking it by setting the object (with shadow) to “transparent” … but that gives you a transparent shadow (!) as well. So that didn’t work!