1. Do you mean the floor grid or the Ground Plane prop?  For the latter, just turn it off in the menus: Display->Guides->Ground Plane (Ctrl+G).  For the former, you cannot change that at all - it represents Y = 0 and there is no changing it.  The solution: change where your figure and world are in relationship to the floor.

2. Are shadows enabled for the renderer (Poser 4 or FireFly)?  Are Cast Shadows enabled for the figures/props in the scene?  Do the lights consider shadows?

3. You can change the background color in the Render Settings.

4. Use a (LARGE) sphere and map a night sky texture image to it - or get BagginsBill to show you how to make one procedurally. ;)  To avoid darkness - especially occluded lights, make sure they are point or spot  lights and inside the sphere.  Infinite lights will cast shadows from the sphere internally on everything.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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