Move CSS files into css directory

Problem

Seven is the only theme which hasn't a 'css' directory where the CSS files are. Is there a reason for that? I thought that all modules and themes should have the CSS in a 'css' directory.

I cannot create a patch right now because there are some issues with the CSS of the new installer:
#2032895: Follow-up use better technique for vertical centering
#2033137: New installer sidebar not properly RTL