If you want to make it night time, change the sky textures in the bitmaps section to black if you need texture editing help download quaglinoman's tutorial.
To make it darker, go the fog section (make sure you have all the latest HMT v3 plugins) and change the numbers to make it black. I think you make them all zeros but im not sure, so try different numbers. Then increase the fog thickness and itll be dark.
Also go the lens section and replace the afternoon sun thing with like a warthog light or something. This will get rid of the sun.
I suggest doing all three of these for an awesome night time effect.