Did you create a zip file of the theme? Try unpacking it. The mistake most often made that all the files are a level to deep. When unpacking you should see the login.html and index.html in the main directory not in a subdirectory.
Regards,
Ronald
Indeed, so I found out. Quite confusing that it did pick up changes in theme.css, but not login.html. I have put it all in a zip now and hope it will work.