Something to consider:

The fact that traditional art galleries use a white background, as shown in those links, is not a valid argument because the process to create traditional versus digital is often quite different and has different dynamics for viewing pleasure.

Rendered art, which is a majority of Rosity's works, are built from darkness and then lit to create the mood. This is much like film and theatrical presentations. We light the actor, the sets with various visual effects while keeping shadows (which are inherent in the renders from the absense of light) to create foreboding. In film, we see such visuals in a darkened theatre, so that we feel the moods and atmosphere in that type of medium. Can you imagine watching a dark light sabre scene in Star Wars in a well lit room? Next time Star Wars is on, watch the same scene with the lights on and without the lights on and you'll see what I mean. Rendered art has the same dynamics and thus needs to be displayed on a dark background.

Traditional art, on the other hand, is the opposite. It starts in light and an artist darkens that for mood. In the links you provided of those galleries, I noticed that most works were of midshade to light and only a few were darker oriented. And even those were not the dark images that you see on Rosity for the most part. That is why such sites can get away with a white background.

Factor in layingback's excellent analysis of monitors and it isn't difficult to see why this is an issue for so many artists and why Rosity should reconsider their stand on the default colors (at least IMO)...

ShadowWind