It's not making the game darker. It just broke the HDR, like overexposing, which you have to lower exposure to maintain brightness balance. And the question, sure. 0.5, 0.25, 0.1 and vanilla GI influence. Using relaunch sky. I've found another workaround that should let you keep the original exposure values. Spoiler Above image uses relaunch exposure values and E3 sky with custom sky light intensity.