Here's one way to do it. Add the following turnscript to your game:
<turnscript name="dimmer">
<enabled />
<script><![CDATA[
if (game.lastoutputsection > 1) {
JS.eval ("$('.section"+(game.lastoutputsection-1)+"').css('opacity', '0.5');")
}
]]></script>
</turnscript>
It sets the opacity of the preceding output section to 50%, which dims the text (shows black through). You can adjust the opacity percent by changing the 0.5. A value of 1.0 would be 100%, 0.0 would be entirely transparent, etc.
A full sample game is attached.