'Relative body' css selector

I am creating my own theme with the Genesis starter theme. I added a background image to my css body selector in the form

body
{

background-image: url('images/Background.jpg');
background-repeat: repeat-x;

}

Everything was working fine until i tried opening my webpage yesterday and I saw that the background image had dissapeared. Using firebug, I checked the code of the page.com and i found that the 'body' selector shows 'relative body', i.e.

relative body
{

background-image: url('images/Background.jpg');
background-repeat: repeat-x;

}

I went back to check the code in my page.css, but it still says 'body', but firebug says 'relative body'. When I remove the 'relative' in firebug, the background re-appears. I am confused as to what to do. Any help will be appreaciated.