Regarding the last post. I was able to use the subthemed css file with the system generated name of 5318d6b673b817f66f8dc5a5b1a54d56.css. I might try to rename the file into something a little easier to remember but for now I think that I’m good.
I believe that this file will be stable as long as it is IN USE when I clear the cache, which I have to do all the time even though I have all the performance enhancers disabled right now. I have seen that the system generated CSS files used by CTools are purged when the cache is cleared if the files are not being used by any Panel Panes. Fun huh?
04/30/13 update – I removed the styling from all the Panel Panes using it and then cleared the cache. This did not remove the style from the Stylizer or interfere with the way that I subthemed it. And changes I made to the CSS file that I subthemed (the 5318d6b673b817f66f8dc5a5b1a54d56.css file) were present.
So, this appears to be a viable way to add CSS to Drupal Panels in a fairly flexible way. Very nice!