Opaque or Transparent Backgrounds via CSS
Some of the cooler looking sites sometimes use transparent layers/div’s to achieve the look. One method to get a transparent background is to use a png image with partial tranparency (see a discussion of that here). Unfortunately, Internet Explorer doesn’t much like png’s unless you go a bit to make them work.
You can also make Semi-Transparent Backgrounds with CSS and they are supported on FireBird/FireFox/Mozilla/Netscape, Internet Explorer, and Konquerer/Safari. I don’t currently run Opera, but I understand they are not currently permitting it.