Via developer mode you can change less, css, twig, etc files. Save works ONLY if the file is in the root of the package. If in a directory, you will get an error that it is not possible to save changes.
This limits the functionality of the developer mode :(
Workaround = download zip, change, upload zip.
I hope it will be fixed some day.
Answered by richardhealy
See post in contextHi!
The developers here have had a look at the issue. This should be fixed for you now! Let me know if you're still having trouble.