I dont completely understand what you are asking, but Firebug for Firefox allows you to edit the css of a page on the fly so that you can see the results immediately. This really is helpful when you are trying to modify your interface and dont want to keep refreshing the page. Is that what you are looking for?

there certainly is, try this: http://usejquery.com/posts/8/brosho-design-in-the-browser-jquery-plugin

I'm also looking for a similar plugin that should allow the end user to apply css changes on the fly and add these changes as inline to the HTMl tags. So that one can save all the changes once a user is done. Example here http://www.cssportal.com/css-style-editor/ or when you create a page on about.me