Well, you have several way easy ways...
A general one is to edit your user.setting and modify the value of "Gamma"... it will modify the luminance of all you see on screen... http://upload.wikimedia.org/wikipedia/commons/9/96/GammaCorrection_demo.jpg
A other way, is to simply disable bloom in your setting, it greatly reduce the light emit by luminous object
Finally, for more specific work, you can simply edit the texture and make it more dark... by example, only edit texture from green star...
There is maybe some other ways ( have not test them, so i am not sure about it )... by example in star_xxx.mesh, you have "Emissive ff000000" ... in theory, if you tune it, it can change the amount of light ( for test purpose, try the value "Emissive ffffffff" like for ship ) ... a other possibility for the skybox, edit the alpha channel from the texture, it seem to be related to the luminosity of the texture...
For human eyes, luminosity in RGB color space is calculated by the following formula : L = 0.2126 R + 0.7152 G + 0.0722 B ... well, shader code have already a lower value that these needed for a normal human eyes... by example for the skybox bloom, you have "float intensity = dot(oColor0.rgb, float3(0.3, 0.59, 0.11));" ... since human eyes is the most sensible to green light, you can try to chance a little the 0.59 value... not sure that it will work because i have bad experience in the past with editing/creating shader code for sins...
