Why aren't CSS files served? There is nothing in sites/default/files/css
A few more things to check when you have issues like this:


Any noteworthy issues on the Status page? (admin/reports/status)
Does saving the File system page generate any errors? (admin/config/media/file-system) 
This sometimes indicates permission or ownership issues in the file structure, which could keep the aggregate CSS and JS files from being generated?
Have you checked the error log for any noteworthy messages? (admin/reports/dblog)


Clearing the cache is a good place to start, but I see that you already did that. Try checking the error log immediately after clearing the cache to see if anything pops up.