You can add the jquery library, combined with jquery ui - if you don't use it already - and use the switchClass method.
All info here : http://jqueryui.com/demos/switchClass/
It will only take 5 lines to do what you want :
Place jquery en jquery ui in the head section of your page (2 lines of code).
These are the remotely hosted files, you can copy/paste the code 'as is'.
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>
<script src="http://code.jquery.com/ui/1.8.18/jquery-ui.min.js" type="text/javascript"></script>
And then, at the end of the body, place a script that contains these three lines of code :
$(".yourbutton".click(function() {
switchClass('.currentclass','.redclass',500)
// transition from .currentclass to .redclass in 500 milliseconds
});