I prefere seteffect:
NPC Code:
seteffect red,green,blue,alpha
Since you want to make the room darker lets use black.
NPC Code:
seteffect 0,0,0,alpha
The red, green, and blue have been replaced by zeros because using a RGB coloring system 0 red, 0 green, and 0 blue is black.
Now for the alpha, the alpha is the degree of intensity the color is put out.
0 being the most and 1 being the least.
This means you vary the level of darkness with decimals between them.
NPC Code:
//#CLIENTSIDE
seteffect 0,0,0,1;
This makes the level completely visible and it your starting point.
Now to make it half between dark and light you would change the alpha to 0.50.
NPC Code:
seteffect 0,0,0,0.50
Now just adjust the alpha to your specifications and enjoy.
This is //#CLIENTSIDE.