Thanks to the order in which CSS files are registered and then rendered by the CssLink control, our good friend core.css (/_layouts/1033/styles/core.css) will always override your background color with background-color:#FFFFFF; (line 7).
Usually I'd recommend using !important to keep the background, but unfortunately Internet Explorer 6- doesn't support it properly.
To resolve, it unfortunately means adding a manual or after the CssLink control.
Also, those behavior attributes (core.css: line 12, line 1973) won't pass validation so I hope they don't render to the end user. Don't make me come down there Microsoft.