Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Can you explain a bit more about what you're talking about? What do you mean by "screen"?
 
Screen

It seems that the default screen background is black. In windows it's possible to change the default color. I can't find how do that with my mac. Hope that's clear.
 
You can change the Desktop image in System Preferences --> Desktop & Screen Savers. The default should be the pic in the Apple Images category called "Aqua Blue."
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.