Open yer file location for Left 4 Dead 2, open the folder called left4dead2, then open the cfg folder.

Look for the file called "video.txt" at the bottom of the folder, and open it with notepad. (Or any equivalent application.)
(Edit- It seems you can do this on videodefaults.txt along with video.txt and not have to do the last step at all!)

Once ya open the file, you should see somethin like this.


At the bottom of the file, create another line of space to paste this line in, above the "}"
"setting.mat_tonemapping_occlusion_use_stencil" "1"
It should look somethin like this.

Now close and save the file. On to the next step!

Click on the video.txt file (don't open it), then right click, and open properties. Check the box on Read Only, and apply and close it.
This will make it so your settings will not be changed without you changing them. For most people, the setting you just added into the file will get removed and your maps will keep being dark, if you don't apply read only.

Once yer done with all that, you're free to go and enjoy your game! xD
Lets pretend I didn't take 4 years to finally edit this guide XD I am very sorry for how long it took me to edit it. I procrastinate too much
ANYWHO, Enjoy your game, and pop em zombie heads!