Distro: Arch
Plasma ver: 5.20.1
Display Adapter: GeForce GTX 1650
Display Drivers: nouveau
Open GL ver: 4.6 (Compatibility Profile) Mesa 20.3.2

In the desktop effects, I have the desktop cube enabled (obviously), no matter what I do to change either the background color or image, all I get is a black background.

I have searched google using terms like "plasma 5 cube background not working", "plasma 5 skydome not working", "plasma 5 upstream patches cube background", all to no avail.
Can someone please point me in the right direction?

I have the same here.

Operating System: KDE neon 5.21
KDE Plasma Version: 5.21.2
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2
Kernel Version: 5.4.0-66-generic
OS Type: 64-bit
Graphics Platform: X11
Processors: 12 × Intel® Core™ i7-9750H CPU @ 2.60GHz
Memory: 15.5 GiB of RAM
Graphics Processor: GeForce GTX 1650/PCIe/SSE2

A Bug is reported: https://bugs.kde.org/show_bug.cgi?id=430874 - desktop cube background always black