SPSkin

A very good tool to create and edit SharePoint CSS is http://james.milne.com/SPSkin/. You just have to go at this page and load your existing CSS there (when you press CSS button) and after that you can modify it easily with the options you have in the left toolbar. This is very intuitive and it also have a lot of predefined skins for SharePoint (a few hundred). When you'll finish to customize the CSS there, you'll have to press the CSS button and to Copy / Paste it to your SharePoint. And that's it! You now have a brand new SharePoint skin!