Hi Rob,
Feature specific style sheets are problematic for the very reason you mention. I haven't really found a smooth plan for copying them to skins and in theory they probably are different for different skins.
My recommendations are:
1. avoid using a feature specific stylesheet if possible. Its really only needed if a lot of additional css is needed to style the feature and in this case putting it in a separate file helps keep the main css file smaller. Thats the only reason for doing it. Best if you can manage to style it using existing css class names that are already in use commonly accross features. The idea of skins is to have a consistent look across features so making use of existing classes helps with this.
2. if the feature does require additional css classes note what they are in the documentation or a readme file but just pick a skin to work with until the feature is mostly complete then review how it looks in other skins. Ultimately when you get the survey feature completed I will help go through and make all the skins that ship with mojoPortal work with it and I will implement the other data layers as well if you don't end up doing them yourself.
Cheers,
Joe