For the Darkness MOD version, you'll need to download Darkness MOD separately.
After I reached the 6-7 mission, I started to get a little annoyed that the Weapons ran out of ammo way to fast, and that some of them, like the Missile Launcher didn't give you a long advantage over zombies. So my typical modding bar started to grow, which basically ended in me looking if there was a way to mod the game. I turned out very surprised to find a lot of the content very easily accessible without any sort of 3rd Party Tools (Except for a Image Editor that supports .DDS , but no worries).
So I started editing the weapons and also made Zombies less reactive to you until your very close, but they'll follow for a greater distance.