<http://www.templatemonster.com/demo/39055.html>

I've seen bunch of websites having those natural gradient on their backgrounds while surfing the internet. The thing I do not understand is... how did they do that? I don't think it's a photoshop-created image file cuz the gradient seems like re-sizing naturally and properly according to the screen size of my web browser... So, I thought it must be CSS... but I cannot find which code is actually doing the job :/

Could someone tell me how it's done?