how to change the back ground color

There are a number of skyboxes which are randomly selected when you load a map/menu. If there's one you don't like you're free to mod it - just copy the texture folder in Sins into a new mod and delete/edit the textures at will. : )

I wondered the exact same thing, I'm fond of the purple  skybox colour, reflects wonderfully off of Advent ships

I actually quite like the coloured sky boxes off Kinda gives more of a deep space feel to it.  Does, however, make keeping track of things in your grav well a little easier.  More contrast and that.