The following site is always helpful for learning things - http://www.w3schools.com/css/
It does have introduction type levels and some more advanced level CSS. For myself, I've only ever learned the more advanced CSS when I need it -usually it's trying to do something specific. A quick google search and there's usually an answer.
I'm not aware of any sites that cater to the mid-advanced CSS levels. Most places do all in one type setups from what I've seen.