Ok, let breaks this down.
Quote: "The scene is decidedly more dark! It's a good result."
Nice
Quote: "but I aspire to make a more dark ambient to show up the lights."
Sadly, I can't make sense of this.
If there are currently no lights in the scene; the scene should be pitch black. This is achieved with:
Set Ambient Light 0
Color Ambient Light RGB(0,0,0)
Color Light 0, RGB(0,0,0)
Assuming you made additional lights already; play around with the range of those light until you are satisfied. Use:
Set Light Range [Light Number], [Distance]
The important thing to remember is that Darkbasic's lights are hardware Directx lights and are therefore Vertex lighting.
If the model is low polygon the object will be lit incorrectly with spot or point lights, Directional lights however tend to work effectively with Vertex lighting. The alternatives are texture baking, lightmaps or a per pixel lighting shader.
Quote: "Besides, is It normal that my mushrooms became white when I use this code?"
"Hide light 0" only affects lighting, if your mushroom had a texture make sure it's applied correctly. Or fall back to a earlier version of your code and add "Hide light 0".
Quote: "But if I set the flag to 0, the objects will be not influenced by any light. I want that they are lighted, otherwise there aren't any performance of lights. "
In that case don't use "Set object light". "Set object light" is best used with lightmaps where Darkbasic’s lights are not to have any influence on the environment.