The forum is wrapped with a DIV with yafnet class - find in theme.css
add the background-color
Code:.yafnet {
font-family: Arial, Helvetica, sans-serif;
font-size: 9pt;
color: #3B3b3b;
background-color:#aabbcc;
}
outside of the forum you can add background - color to the body
Code:body{ background-color:Black}