Conditional userchrome.css?

Sorry, I know that this is slightly off topic.  But I've discovered I
can do what I want with Mozilla by using userchrome.css rather an XUL
overlay which is still totally incomprhensible to me (mainly because I
haven't found a code example that matches my particular need).  So -

Is there a method of applying a different set of userchrome.css
entries based on what theme is currently selected? (In other words, if
I'm using Modern then apply section X of userchrome.css, and if I'm
using Classic apply section Y?)

Also, is there any way, via an XPI script, of determining the current
user's Mozilla preferences directory?  (I'm assuming not because of
salting and security, but I thought I'd ask.)