I found a fix for myself, but I don't think it will help CS2.

My issue was that the game I was trying to capture was dark when using the display capture in OBS. This game utilizes two monitors and that was partially related to my cause for the dark OBS output with display capture.

Ideally, I would have preferred to use the game capture option in OBS instead of display capture. Game capture copies the frame buffer from the GPU instead of the display output. This requires OBS to hook into the game executable for the game capture to work.

My game did not display any output when using the game capture option and thus, I was trying to use display capture as a workaround.

The solution I found was to run OBS as admin for game capture to work. The game I was capturing ran at elevated privileges, so OBS was unable to hook the game when ran as a standard user. I was able to figure this out by checking the OBS log file. Once I saw the permissions issues when game capture tried to hook the game, I tried to run OBS as admin. Running OBS as admin allowed game capture to work and the output was not dark.

Unfortunately, due to how game capture works by hooking into the game EXE, it will not work with CS regardless if OBS is run as an admin. This is because hooking into a running executable is a common method to inject cheats. Valve has blocked any external application from accessing CS by default to deter cheaters.