Change gamma ramp
You can change overall ingame brightness with slider or buttons.
Or apply special fullscreen effects...

Try it and You'll see.
Place DLL in your plugins dir, cut WDL code and write it down to your 'main.wdl'. Make shure your game started in fullscreen or press ALT+ENTER, then play with PG_UP and PG_DN keys.
It's hard to explain - my english vocabulary is about 5 words
Sorry.