It appears that in V7, when transparent background is selected, it’s not possible to change the background color until first unselecting it (same for gradient as well).
While this might seem like an odd thing for some, looking at these renders, you’ll realize that it’s still important to be able to change the color.
Thanks for checking this.
All development is being done in Rhino 8 at this point and only specific regressions, crash bugs, and SDK enhancements are being backported to Rhino 7.
-wim