Hi Lawrence,
That code has actually been removed recently. StyleSheet is a control that was used in mojoPortal a very long time ago but was replaced with StyleSheetCombiner. If you need additional CSS for a custom feature I suggest create a css file in your skin folder and include it from style.config.
If you are just trying to make your feature styled with standard styles like most features you may want to look at the latest example code in our Hello World tutorials (or our Codesmith templates) which has been updated to show the use of our new set of container panels.
Hope that helps,
Joe