That error means that the file permissions on your server are blocking the theme from creating files it needs for the admin.
The easiest way to fix it is to use an FTP client like
FileZilla to set the permissions of your wp-content folder to a minimum of 655. If that doesn't work, you may have to go as high as 766, or at a last resort, 777.
Would I edit the folder only, or all the contents within the folder? Changing the folder only does not remove the error, but I'm concerned changing permissions on all the files in the folder will cause problems.