Clock you should check shoutbox.php. I know that the css code is embedded in the php. Look for references like
HTML Code:
<style type="text/css">
#specialbox{
border: 1px solid gray;
width: 600px;
background: #FBFCFA;
font: 11px verdana, sans-serif;
color: #443;
padding: 3px; outline: none;
} bla bla bla
and modify it to your needs.
Cheers!