You can add a Internet Explorer (external link) specific stylesheet to your WordPress (external link) theme using the following code:

<!--[if lt IE 6]>
   <link type="text/css" href="<?php bloginfo('template_url'); ?>/ie6.css" media="screen" rel="stylesheet" />
<![endif]-->

This will link to a Internet Explorer 6 only stylesheet using conditional comments, the ie6.css file should be located in the same place as the standard css file.