The css is cached both in the browser and on the server. To clear the server cache just touch Web.config by typing a space in it and saving it. This will recycle the app and clear the cache.
To clear the IE browser cache I find sometimes it does not clear the cache if the page is still current in your browser. If you navigate somewhere else like google then clear your cache then come back to the mojo site and it will really be cleared.
Hope it helps,
Joe