Opera requires media="projection" for fullscreen. So you need to do media="screen,projection".
The reason is that fullscreen mode in Opera is special. (Note the lack of toolbars.) You can actually use it to do presentations like Powerpoint slides. A site can also have two completely different styles depending on whether it's fullscreen or not.
However, I am one of several people who have campaigned for Opera to fall back to the same styles as normal mode when a fullscreen stylesheet has not been created. A majority of webmasters don't know about Opera's fullscreen mode, so their sites simply lose styles when you hit F11.