OR go into your admin cp...
Goto Styles > Modify
Then in the Category Backgound Color box change the hex to:
PHP Code:
#000000" background="{ imagesfolder}/catbg.gif"
Note: You can change the #000000 to suit your look as the page loads... Also removed the gap from inbetween the "{" and "imagesfolder" and of course you have to make your own catbg.gif gradient
Regards
- Mist