In my Opera 7.21, and possibly other browsers. when I switch the browser to full screen browsing (F11) it stops processing the page CSS completely for any installation of Wordpress I have seen so far which has two different CSS for "screen" and "print".
Solution:
Opera 7.21 treats fullscreen not as a 'screen' medium but as 'projection' medium. Unless you really do want a special CSS for this case, you should replace the <style type="text/css" media="screen"> line in your index.php with <style type="text/css" media="screen, projection">.