CrazyAce
Prince
This concept is very easy, once you have the image you want to use for the stars background just convert it to stars.dds, just remember the image size is 512x512.
Unpack and place the stars.dds file to:
...\My Games\Sid Meier's Civilization 4\CustomAssets\art\Interface\Main Menu
Unpack and place the stars.dds file to:
...\My Games\Sid Meier's Civilization 4\CustomAssets\art\Interface\Main Menu