Caroig,

Use just one .css either one skinname for each skin or skin.css with all.

Personaly I use skinname.css  the .css will bee smaler that loads faster. If it just a few differences I will use skin.css.

Of course you can include the generic .css file in your skinfilename.css using the CSS import directive:

@import url("generic.css");

I think this import is a textual include, so the generic.css is probably not cached, but it will make CSS maintenance easier.