Due to this change: http://svn.wikimedia.org/viewvc/mediawiki?view=rev&revision=38116
Opera in fullscreen mode, does not use CSS at the moment. In Fullscreen mode, opera uses the "projection" media type, but it is not provided anymore. Since there is no "all" media types stylesheet, no styling is used.
This raises an interesting issue btw. We also have 0 CSS for aural, braille and other modes.