Or you can set the ambient light in the build game settings. It's under global script.
Or you can use the following. Open notepad, paste this and save it in your scriptback/user folder as lowlight.fpi. The make a dynamic light and set its range as 5 and its AI main script to lowlight.fpi
NOTE: This is not my script. I know nothing about scripting. However i found it a while ago on the forum somewhere and dont know the author. Please say who you are so correct credit can be given!
;Artificial Intelligence Script
;Header
desc = low light
;Triggers
:state=0:state=1
:state=1:ambience=0,lightrange=1
;End of Script
Fujitsu-Siemens "Scaleo-P" with Windows XP OS, Pentium 4, 3.2GHz Processor, 512MB RAM, and a 128MB ATI RADEON X300 Graphics card.