(Visiting the link, it says it isn't designed for my device - Android, with very latest FF/Chr browser versions - but <something something> collection to run elsewhere, or try anyway (which I didn't, but will when I have time). Paraphrasing badly, from my visit a couple of hours ago. I'm away from all Windowses for the moment, even trusty-old 32bit, so was not in a rush.)
All-black
sounds like completely null texturing being applied[1], which means it isn't even finding "that subtle shade of blue" and applying it. Though as I'm not entirely sure how Godot does this, I'm going by my use (and abuse) of renderers such as Blender, which I think was mentioned and may therefore be a library component. Either that or a completely invalid viewport upon the scene (like the undefined main screen). The reasons for either could be complex or just the failure to download one single package of resource data. Knowing more about what does/does not render (even outside the workshop scene) would be my next question, probably.
Was going to have a look myself, but haven't yet (as per introductory paragraph) and I'm just guessing about how 'scenes' are handled and swapped in/out, so not sure I'm saying anything useful. But when has that ever stopped me before?
[1] Can't produce meaningful specular shading/etc, because any such transform of RGB(0,0,0) still produces RGB(0,0,0). Though if that 'sun' I see in the screenshot is defined as a spotlight-entity, you'd expect to see that (as baseline RGB(255,255,255) in direct/diffuse view) even against a #000 sky.