Brief - feed reading extension for Firefox

To disable the background image, use the following appropriately in Brief's CSS Editor*:
background: none !important;

Link to default style (so you can see where the background CSS is):
http://brief.mozdev.org/default-style.css

Of course if you'd rather use your own background image instead of disabling it altogether*:
background: url('[full address to your background image]') repeat-x top right !important;

* Brief by default owns all style rules, so you'll have to overrule them with !important in Brief's CSS Editor. The only exception to this is if you're editing Brief's internal files in order to replace the default stylesheet with a stylesheet of your own or modifying the default stylesheet.

You wouldn't have to tag !important on things in either case.