The background is the environment set. just check the texture/rgb folder for files witch start with "env_" and enter the name here without the last number (env_red_dust1.tga --> env_red_dust)
The starfield odf defines the rules how the background starfield is created. stars_default.odf, stars_frozen_dust.odf, stars_glow.odf and stars_starlight.odf are the common settings.
The stardate just defines the starting date of the battle for the admirals log

I want use env_blue
must i also change the starfield ?

---edit--- - -
a I have found it