I need help with modifying css in 2.0

I would like to modify my 2.0 wiki to make it less cluttered. I have looked at the pbcss wiki which gives information on modifying 1.0 wikis but I do not know what elements can be modified in 2.0. For example, it would do me no good to modify the header if no header tag exists. I am particularly interested in making changes in the sidebar. I found information on how to remove the comments box, but so far that is the only modification I've been able to make. Any help would be appreciated.