sem-admin-menu plugin Fine Tuning the Page

I’ve been using the admin-menu plugin for more than a year at its default settings. Today I dropped a skin.css file in to customize the bar across the top of the page (as the plugin readme suggests) and it looks good, except now the transparent bar overlaps the top of the header image and obscures the top navigation menu. So I would like to just add some empty space at the top of my page to compensate for the menu-bar overlap. In other words, just slide the page down about 20 px or so. Is this a simple mod?
see it here: http://guitarchasers.biz/media/
Thanks.