javascript distorts font

I've noticed the font is not displayed clear on my website. It is due to the javascript but I can not find the problem. Does anyone know what is wrong?

The border of the font开发者_Go百科 is not straight. Looks something like cleartype not being set. The problem is in mu FF4 browsers. Font-weight is definitely different when the javascript is turned off (less).

Edit: I see what is the problem the Jquery background function. Does it overrule all of the css body statements? Is there a way to just add the background only to the body?

$(document).ready(function() {   

 $.fn.smartBackgroundImage = function(url){
  var t = this;
  $('<img />')
    .attr('src', url)
    .load(function(){ 
       t.each(function(){ 
          $(this).css('background', '#16171e url('+url+') right bottom no-repeat fixed');
       });
    });
   return this;
 }

var viewportHeight = window.innerHeight ? window.innerHeight : $(window).height();
var viewportWidth = window.innerWidth ? window.innerWidth : $(window).width();
jQuery.post('/v3/ajax/portview.php', {height: viewportHeight, width: viewportWidth});

if (viewportWidth>950) {
 $('body').smartBackgroundImage('http://sionvalais.com/gfx/berge.jpg');
}

});