If you create a bookmark in chrome (not sure if other browsers allow arbitrary javascript execution from a bookmark) with this content, it'll kill the background when you click it:
javascript:(function(){$('body').css('background-image','url()');$('.container').css('background-image','url()');})();
Or better yet.. If you like SO's look you can use this:
$('link[href="http://cdn.sstatic.net/programmersmeta/all.css?v=f078930b28c8"]').attr('href','http://cdn.sstatic.net/stackoverflow/all.css?v=0f0c93534e2b');
and it will change the entire theme of P.SE to SO's, or you can try any other by swapping the v=blablabla query string variable.