background-position: center-bottom;

Im using raindrops theme, now i have designed a wallpaper which is best viewed when the position is fixed, center-bottom. Now the options in the menu gives me only top-center,left,right to choose.

I want to put this in the CSS, in the webmaster tool provided by IE it works fine, change it (temporarily) to background-position: center-bottom; and the wallpaper starts on the bottom.

Where do i put this code? I have tried every piece of code with a body provided, but as i thought it is made by some piece of PHP, and not an existing CSS file so i can’t change this.