Youtube!? Here I was just noticing that they've changed to a pure grey background. Can't remember what they used to have. Is it one of those google-style experiments where different people see different things?
There's a little bit of a gradient along the bottom. Is that what you're asking about? If you load up YT's primary stylesheet-- link in the front-page source-- and search for the word "gradient" it goes on for line after line after line.
You can create patterned background using Photoshop software. First you make a nice pattern using the selection tools and then load the pattern.Then select background layer and using pattern clone stamp you can create a patterned background.