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