Okay, I had this problem too, and the files aren’t corrupted. The problem is that your themes folder (where your themes are in) is protected from reading! In your FTP Manager, right click the folder and press Properties or Protection, or something along the lines of that. Then make sure the “read” checks are all checked. That should fix it!
I had that problem with my plugins too, and did the same.
You can also do what doodlebee said, and reinstall WP, because that also fixes the protections to defaults.
