Apply different CSS classes to message and comment bodies

I’m in urgent need to make a minor design change to our RT instance.
The gist is to keep the class=“messagebody” for messages and implement class=“commentbody” so that comments and messages look different. Is that possible without major code changes?
RT User

I remember when there was a template override system, there was a directory were you could place a Mason (a web framework for Perl) template with the same name of on of the original RT template and your version would override it.

Looks like there is a theme system now so just copy the default theme and change it to your hearts content.