Use Prims....
The effect of the following process is that the room appears to be dark and as you walk closer to the center, the room becomes becomes brighter. There is another techique at the end of this message that does not use as many prims.
*First Technique
Rez a cube...make it black, phantom, and, say, 70% transparent. I size the cube to be as high as the room or field of view and flatten it to aout 0.75m.
I position the cube so that it is about 10m away from a point of interest (bed, sofa, etc) or at the entrance of a room. The first cube is the farthest away from the 'point of interest' (10m in this example). I rez another similar cube about 2m closer to the point of interest and reduce its transparency by 10-20% and continue that until I am maybe 1-2m away from the bed or whatever (having the last cube at about 10% transparency).
NOTE: You cannot, unfortunately, surround your bed/sofa in a cube like this because you would not be able to click anything (the cube gets all of the clicks) but it still is a nice effect.
If you want to apply this to a whole room, you'll have to surround the point of interest in these cubes to make it feel consistent, or resize the room, or create a hallway that leads to the point of interest.
*Second Technique
You could make the whole room feel darker by making the walls black and highlighting the walls with white lights (transparent texture) in some places. Take a look at Starax Statosky's shop (search for Starax Statosky and go to the store listed in the profile) - the shop is completely black and uses particles and interesting textures to achive a feeling of darkness. Of course, using the transparent cubes in this case does not make sense because the whole room is already dark - although you could make the entrance like that to avoid outside 'light' from entering when you open the door.
You'll have to use furniture that does not have any white in it to avoid your eye being drawn to the stark white lines. The technique makes the room a little disorienting since you cannot see the walls or floor. Add a picture, carpet, and other elements to give your guests some points of reference so that they don't end up walking into a wall

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Good luck - let me know if you need help with this - I am around in world starting at about 7pm SL time.