The OXR Toolkit is unable to really decrease the brightness. Why? Because in the game if you have very bright clouds (e.g. in an A320) outside and the cockpit is dark, then the clouds are completely white (uniform color, 100% white). You can decrease this brightness to say 80%, but all 100% bright parts before only get darker to 80% and you see no structures in them. This must be done in the game engine. It is like an exposure of a camera. In Drone Mode, this already works.