This has caught me out twice now, trying to upload my custom theme file. I activate publishing features thinking this will fix it, it doesn't. I then go to the URL for themes "/_catalogs/theme/15/" and upload my SP colour file, i get the error “The documents cannot be uploaded because different permissions are needed. Request the necessary permissions.”
After googling around, it turns out it's nothing to do with user permissions and you actually need to enable scripting on your site collection, see here. If you change the setting manually it will take 24 hours, but you can use my C# code or PowerShell from this blog post.
Web Designer Galleries before changing setting:
Web Designer Galleries after changing setting:
- Web parts
- List templates
- Composed looks