You can try adding...
#shadowbox_content {background-color:#ffffff;}
...or...
#shadowbox_body_inner {background-color:#ffffff;}
...or changing...
#shadowbox_body {background-color:#000;}
...to...
#shadowbox_body {background-color:#ffffff;}
...in your css.
In IE7 the sites look fine - but that is because the iframe is not transparent in IE7, whereas in Firefox the iframe
is transparent (which it actually should be in IE as well!) so sites that don't set a background colour on BODY will have a transparent BODY and hence have a black background (from #shadowbox_body).
Combine literacy with curiosity and a whole world of information opens up to you