Unfortunately, currently the HTMLFrameMojit doesn't allow you to do add a class to the body element.
What you can do is copy the HTMLFrameMojit that comes with Mojito source into your app's 'mojits' directory and then modify the views/index.hb.html, so that the body element has the class 'yui3-skin-sam'. HTMLFrameMojit is just another mojit (you could create your own frame mojit, too), so your local copy of HTMLFrameMojit will simply override the HTMLFrameMojit that comes with Mojito.
The following instructions might speed things up for you:
1. git clone https://github.com/yahoo/mojito.git
2. cp -r mojito/lib/app/mojits/HTMLFrameMojit <your-app-dir>/mojits
3. cd <your-app-dir>/mojits/HTMLFrameMojit/views
4. Edit 'index.hb.html' so that the 'body' element has the class 'yui3-skin-sam'.
5. Run your app.