You can create your background tile in any graphics software you like.
Make sure it tiles horizontally without any obvious joins. Create the tile the exact height of your web page, eg: if your page is 1024 pixels high make your background tile 1024 pixles high.
Import the tile into Xtreme or Web Designer then delete it off the page.
Open the bitmap gallery, select the tile and press the 'background' button.

(If you use Xtreme or Web Designer, enable grid snapping. Make sure your tile is drawn in whole pixels. I exported as 256 colour PNG.)

Done - that's it.

You can get some very nice textures for backgrounds here