Flat colour background with shadows in Flamingo

The option I used a lot in Flamingo nXt is a solid colour background (or a gradient) and in the environment the groundplane enabled and the alpha channel enabled.
For some mysterious reason these options are no longer available in Flamingo 5.

I still want to use this option in Flamingo 5 (Regrettably, I cannot use nXt any longer on Windows 10. If I could I would not hesitate one moment to use nXt)

Please help. Thank you!