Currently the only way supported by the CrystalSpace based PlaneShift to change the brightness of the 3D display, is the manipulation of the brightness of the "Crystal" light source. Where the Crystal doesn't shine, you won't see a change in brightness.
If PlaneShift is too dark for you in certain areas, you need to find a solution outside of the game: Gamma correction for the whole desktop display. This is specific to your combination of graphic card, operating system, and drivers.
Supporting gamma correction in PlaneShift / CrystalSpace via OpenGL functions is not trivial, because it requires operating system dependent OpenGL extensions.