Content background image

First let me say that Tempera is an excellent template and the way you can dress it up in different colors are both easy and powerful. My question takes this a little further: how is it possible to add a background image to the content area (not the background background)?
I can’t seem to find the custom changes in the main css-file so I guess it is kept in another custom css?
Phil @ http://promemoria-music.se/