<style type="text/css">
.textbox_normal { background-color:#FFFFFF; }
.textbox_activado { background-color:#FFFFCC;color:#0000FF }
</style>
<p><input type="text" name="caja" size="30" maxlength="40" class="textbox_normal" onFocus="this.className='textbox_activado'" onblur="this.className='textbox_normal'"></p>
<p><input type="text" name="caja2" size="30" maxlength="40" class="textbox_normal" onFocus="this.className='textbox_activado'" onblur="this.className='textbox_normal'"></p>