Hi again and thank you again!
i managed to get the style with the php to work now the pages load.
the question is how do i define the “is_page” or how it knows which page is it?
<style type="text/css">
#site_left_column div.home_bg {background-image: url(<?php if(is_page('unternehmen'))echo bloginfo('template_url').'layout/side_menu.gif'; else echo bloginfo('template_url').'/layout/side_menu1.gif'; ?>);}
</style>