Okay, that background is entirely outside of the forums control then.
Afaik the preview client draws its own tile, which mostly renders thumbnails inside a white (or very light) tile by default, regardless of rest of it. Even if the dark card is dark, the image area is usually a white square the source PNG gets pasted onto. Transparent pixels = white pixels = invisible white “r”.
The change you did should work, but the external preview-renderers might take a while to pick it up. Should work fast with a brand new invite link though, to test.