File Permissions for Editing Theme Files

If you’re not the admin of the blog then you don’t have a capability to edit those theme file.

but if you’re really an admin and has the ability then the problem in in your server. You have to change the permission of the file directly from your cpanel server.

I think 666 to the actual theme files is appropriate or to the folder recursively
https://codex.www.ads-software.com/Editing_Files#Things_You_Need_to_Know

Brilliant, thank you!