Hi Andria,
The div with the class cmwrapper is used to wrap either comment system and is intended to give you a hook to be able to style the container if needed and as needed for your design.
I will go through the included skins that ship with mojoPortal for the next release and add this to put a little margin there to prevent it hitting the edge.
.cmwrapper { margin:10px; }
but anything beyond that should be decided by the designer. I don't think we should globally push any CSS into the skin other than what is defined in style.config. That would be taking control away from the designer.
You can style it in your designs with whatever works for you and it hardly seems like it is much work to add a little CSS to your skins yourself. If you are finding you need the same CSS in all your designs you can define that stuff in a common file as Jamie suggested and include it in your style.config. But for such a small css fragment it seems easy enough to me to just add it to an existing css file instead of a separate one. Typically a new skin uses an existing skin as the starting point so once you have added it to your current skins I don't see how it is any big maintenance problem for your future skins.
Best,
Joe