i think you can set white color for container background, and then change the opacity of it:
#container{
background-color:#ffffff;
filter:alpha(opacity=50); /* need for ie6 */
-moz-opacity:0.5; /* Netscape*/
-khtml-opacity: 0.5; /* Safari (1.x) */
opacity: 0.5; /* standard browser ,Firefox, ie7, Safari ,*/
}
another usefull link:
http://www.mandarindesign.com/opacity.html
http://css-tricks.com/css-transparency-settings-for-all-broswers/