Hi Lars,
CSS is cached both on the server and by the browser. Recycling the app or touching web.config should clear the server cache. Restarting the browser does not clear the browser cache.
The easiest way to make sure CSS changes are applied immediately is to reset the skin guid by clicking a button under Administration > Advanced Tools > Design Tools Cache Tool
as mentioned in the 3rd paragraph on our Overview/Introduction to Skinning.
Hope that helps,
Joe