You don't need to add a file, you can put the additionjal custom css in an existing file like style.css, if using an Artisteer skin I would not touch style.css (since that is exported from Artisteer) but instead use stylemojo.css

Whenever you make changes in CSS you need to then go to Administration > Advanced Tools > Design Tools > Cache Tool and click the button to reset the skin guid, this will make sure the css is loaded from the server again instead of from the browser cache

Hope that helps,

Joe