open one of the thing .lvl files you want ( depending on what level you want brighter or just the whole game or a mod ) with notepad and then change those stats to these
"FogColour" = "0.35,0.35,0.60"
"FogStart" = "140"
"FogEnd" = "9999999"
"AmbientLight" = "0.7,0.7,0.7"
