Transparent background is actually the default of div boxes : ) That's why, if you have a background colour on the body, you will see the same colour in any div boxes where there's no colour specified.
I'm guessing you want it to be not fully transparent in which case you would use these CSS properties: opacity = 0.5; -moz-opacity: 0.5; -khtml-opacity: 0.5; filter: alpha(opacity=50); The variants are for Gecko, KHTML and IE based browsers.