Edit the "Sun" and "Sky" portions of the mission file. You'll want to change the colours to lower values to make them darker..
A comparison of Bedroom and Bedroom Dark for an idea of what you're looking for:
//--- OBJECT WRITE BEGIN ---
new SimGroup(MissionGroup) {
new ScriptObject(MissionInfo) {
name = "Bedroom";
saveName = "Bedroom";
}
...
new Sky(Sky) {
position = "336 136 0";
rotation = "1 0 0 0";
scale = "1 1 1";
materialList = "base/data/skies/Sky_Blue2/resource.dml";
cloudHeightPer[0] = "0.349971";
cloudHeightPer[1] = "0.3";
cloudHeightPer[2] = "0.199973";
cloudSpeed1 = "0.0005";
cloudSpeed2 = "0.001";
cloudSpeed3 = "0.0003";
visibleDistance = "900";
fogDistance = "500";
fogColor = "0.950000 0.950000 1.000000 1.000000";
fogStorm1 = "0";
fogStorm2 = "0";
fogStorm3 = "0";
fogVolume1 = "0 0 0";
fogVolume2 = "0 0 0";
fogVolume3 = "0 0 0";
fogVolumeColor1 = "128.000000 128.000000 128.000000 -222768174765569860000000000000000000000.000000";
fogVolumeColor2 = "128.000000 128.000000 128.000000 0.000000";
fogVolumeColor3 = "128.000000 128.000000 128.000000 -170698929442160050000000000000000000000.000000";
windVelocity = "0 0 0";
windEffectPrecipitation = "1";
SkySolidColor = "0.600000 0.600000 0.600000 1.000000";
useSkyTextures = "1";
renderBottomTexture = "0";
noRenderBans = "0";
locked = "true";
};
new Sun() {
azimuth = "250";
elevation = "35";
color = "0.700000 0.700000 0.700000 1.000000";
ambient = "0.400000 0.400000 0.300000 1.000000";
locked = "true";
direction = "0.57735 0.57735 -0.57735";
position = "0 0 0";
scale = "1 1 1";
rotation = "1 0 0 0";
};
...
};
//--- OBJECT WRITE END ---
//--- OBJECT WRITE BEGIN ---
new SimGroup(MissionGroup) {
new ScriptObject(MissionInfo) {
saveName = "Bedroom";
name = "Bedroom - Dark";
dark = "1";
};
...
new Sky(Sky) {
position = "336 136 0";
rotation = "1 0 0 0";
scale = "1 1 1";
materialList = "base/data/skies/Sky_Blue2/resource.dml";
cloudHeightPer[0] = "0.349971";
cloudHeightPer[1] = "0.3";
cloudHeightPer[2] = "0.199973";
cloudSpeed1 = "0.0005";
cloudSpeed2 = "0.001";
cloudSpeed3 = "0.0003";
visibleDistance = "500";
fogDistance = "300";
fogColor = "0.000000 0.000000 0.300000 1.000000";
fogStorm1 = "0";
fogStorm2 = "0";
fogStorm3 = "0";
fogVolume1 = "0 0 0";
fogVolume2 = "0 0 0";
fogVolume3 = "0 0 0";
fogVolumeColor1 = "128.000000 128.000000 128.000000 -222768174765569860000000000000000000000.000000";
fogVolumeColor2 = "128.000000 128.000000 128.000000 0.000000";
fogVolumeColor3 = "128.000000 128.000000 128.000000 -170698929442160050000000000000000000000.000000";
windVelocity = "1 1 0";
windEffectPrecipitation = "1";
SkySolidColor = "0.000000 0.000000 0.200000 1.000000";
useSkyTextures = "0";
renderBottomTexture = "0";
noRenderBans = "0";
locked = "true";
};
new Sun() {
azimuth = "250";
elevation = "90";
color = "0.000000 0.000000 0.000000 1.000000";
ambient = "0.000000 0.000000 0.000000 1.000000";
rotation = "1 0 0 0";
direction = "0 0 1";
scale = "1 1 1";
position = "0 0 0";
};
...
};
//--- OBJECT WRITE END ---
The "dark" field in Bedroom Dark makes people turn on their light if they have enabled "Auto-Use Light in Dark Maps" in Advanced Options, it doesn't actually make the map darker.