It is very difficult and non-intuitive to change the media type for a CSS file. The only reliable method seems to be:
1. Ensure that the HTML file has more than one stylesheet and that the one involved is not first on the list
2. Expand the stylesheet to be changed
3. Make a change to a rule
4. Make the required change in the 'Media List' box
5. Move the stylesheet up in the priority list
6. Make a change to the HTML
7. Save the file
Steps3 and 7 ensure that both HTML and CSS file will be saved. After that go back and restore any made changes at steps 3 and 5 that you would have preferred not to make.
The problem was inherited from Nvu so is not high priority to fix.