ok, in your header you wrote
codice:
Rounded("div.contentpaneopen","all","#FFF","#D4DDFF","smooth");
But you applied class contentpaneopen only to <table> element and no one to <div> elements: could be this the problem? If yes just write instead
codice:
Rounded("table.contentpaneopen","all","#FFF","#D4DDFF","smooth");
Another thing: as this page seems to say (http://www.html.it/articoli/nifty/index.html), I see only 4 parameters applied to Rounded Function, not five as you wrote (but maybe is a different implementation of code for joomla)