I' ve just had a look in the base game file and it is about one third of the way down in my lighting.txt.
projectiveShadowStrength 0.73
 # modifiers insideObjects  0 0 0 0
 # modifiers insidePortals      0       0         0 0
 #  modifiers insidePortalSpills 0       0          0 0
   # extra ambient for unlit rooms.
   unlitRoomColor (.3, .275, .275) -threshold .33
   unlitRoomColor (.45, .425, .4) -threshold .4
   flatShade (1, .95, .85) .75  -sims 1.7 -objects 1.3 -indoors 1 # for lighting set to low.
   environment (1, 1, 1) 1
   
   #               max in time (s)   max out time (s)
   transitionTimes        2                 100
end
Then if you scroll down a bit there is another transition Times under night time.
I've just changed both the 2s to 30s and will go and check out what it does.