<div style="border:1px solid #8801AA; background-color:#EEDDFF; padding: 10px 30px ;">
    Rahmen: #8801AA<br />
    Text: #EEDDFF
</div>

<script type="text/javascript">
//<![CDATA[
        function changeFontSize(element,step)
        {
                step = parseInt(step,10);
                var el = document.getElementById(element);
                var curFont = parseInt(el.style.fontSize,10);
                el.style.fontSize = (curFont+step) + 'px';
                return;
        }
//]]>
</script>

<a href="javascript:void(0);" onclick="changeFontSize('css','2');">Increase font</a>
<a href="javascript:void(0);" onclick="changeFontSize('css',-2);">Decrease font</a>
<div id="error" style="border:1px solid #4567BB; background-color:#DDEEFF; padding: 10px 30px ;">
    <p id="css" style="font-size:12px;">
        Rahmen: #4567BB<br />
        Text: #DDEEFF
    </p>
</div>