I had the same problem. And my files were looking perfect, readable and writable, I could open them individually and edit normally, but wordpress would say style.css was not readable.
After a while I realized that my server might have assigned chmod sharing folder permission for all my files and folders (it appears as “rwx”). I simply changed the files (not the folders) to sharing file chmod’s permissions (rw-) and refreshed the “presentation” page. The theme appeared and worked normally!
Hope this helps
