I'm assuming you are talking about artisteer-greenlagoon skin.
I believe that is coming from these entries in the stylemojo.css:
ul.linkitem li { list-style:none; min-height:18px; margin:0 0 1px; padding:2px 0; }
ul.linkitem a { margin-left:3px; font-weight:700; text-decoration:none; padding:2px 5px 2px 20px; }
* html body ul.linkitem a { height:18px; }
ul.linkitem a:hover { padding:2px 5px 2px 20px; }
li.linkitem:hover,li.linkaltitem:hover { border-left:5px solid #4EBF37; }
Hope it helps,
Joe