I'm going to try attaching the PNG rendered over black. I don't think Rosity supports PNG attachments, but it's worth a try.

To get the black background I used "Render over: Black" In  the Render Options, rather than setting the colour from the display menu, don't know if that makes a difference.

EDIT. It worked! :biggrin:

Although the background may look blue, or some other colour in the web page, if you save it to disk you will see the black background.