If you want to make the header background transparent in a particular style, then edit the Stylevar "header_background" in the "Header" section of the Stylevars.
If you wish to quickly apply this change to all of your styles, then add a plugin hooked at "parse_templates" with the code:
PHP Code:
$template_hook['headinclude_css'] .= '<style>.above_body {background: transparent !important;}</style>';