[Jenta]

thats under style manager, edit css
the class name is bginput

not sure if the form element IN THE SHOUTBOX has that class applied
if not just add class="bginput" to

<input type="text" name="message".....

this will affect any text input form on your site

to give u an idea mine is

textarea, .bginput
{
	background-color: #F5F5FF;
	color: #15326E;
	font: 10pt verdana, geneva, lucida, 'lucida grande', arial, helvetica, sans-serif;
	border: #26a solid 2px;
}

you can search for style sheets, css, forms and get some ideas of all the things u can do with form elements