[GoDaddy Mark I] menu covers content

Please check my site before answering: http://dessibodywork.com/
I have been struggling with a menu that covers the content of my pages, especially if the window is not full or if ctrl+/- is used to make the view smaller/bigger.
I can’t seem to be able to make ANY changes by editing the stylesheet (style.css). In fact, the code there contains colors that match the original theme, but not the skin (green) that I use.
Really prevents me from marketing my site.