To make your content boxes so they are clear/opaque:

If you are not using CSS, go to, control panel > site style > site wide style > site layout.
Now in the "content box background" area, click on the color box that brings up the color selector, drag your cursor to highlight the hex number in the box. (Ex. #000333) now just delete the number so nothing is in the box, and hit enter to save. Now you should have clear content box backgrounds.

If you are using CSS:
Just remove the hex code from the contentboxbody element

.ContentBoxBody {
background-color: ;
background-image:url('');
background-repeat:repeat;
color: #dedede;
}

As for the banner, if you are talking about the generic one GP lets you make in the control panel. No I don't see any option for banner background. You could copy the image to your pc and add a background with some editing software.