I ran into this problem while trying to rob WordPress of their proper recognition while setting up this site. I’m hosting on AWS, so I have an SSH connection open that allows me to interact with files through the command prompt. If you are not familiar with what I just said then you should find a different post about this.
Anyways, it’s fairly simple, but if you’re only vaguely familiar with Unix permissions then this might be a useful tidbit. I changed directory until I was in the theme’s subdirectory and ran this:
chmod 746 style.css
If you are dealing with a different file than style.css, you can adjust this command accordingly. This opens things up just enough to be workable, as outlined by the list in this section.