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I don't know of a CSS-only way as Opera 11 is still VERY new.
You can either use server-side languages like PHP to detect the User Agent of the browser or you can use the freely available Javascript solution CSS Browser Selector.
The above solution does not yet include Opera 11, so let's check Opera 11's User Agent string by checking their references. (Actually they have their own article on how to detect opera)
Opera/9.80 (Windows NT 5.1; U; en) Presto/2.7.39 Version/11.00
When you now look at the Javascript of the above mentioned CSS Browser selector you can see it is just reading out the navigator.userAgent and comparing it to many variations - just add your Opera 11 variation and you are good to go (or wait until the developer updates the javascript - or even better, update the script and tell the author about it!).