Puoi fare cosi

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.textboxpagine {color: #000;font:8pt arial,tahoma,verdana,sans-serif;border: 1px solid #000;background-color: #d3f3f9; }
.textboxpagine2{color: #000;font:8pt arial,tahoma,verdana,sans-serif;border: 1px solid #000;background-color: #ffff99; }
-->
</style>

</head>
<body>
<textarea cols="50" rows="20" class="textboxpagine2" onfocus="this.className='textboxpagine'" onblur="this.className='textboxpagine2'"></textarea>
<textarea cols="50" rows="20" class="textboxpagine2" onfocus="this.className='textboxpagine'"  onblur="this.className='textboxpagine2'"></textarea>

</body>
</html>