Today the stylesheets generated by individual themes set an Expires header which allow them to be fetched directly from the cache until expiry. However when the cache expires they have to be refetched, even if they haven't changed. This usually means about 100kB (20kB if compressed) unneeded network traffic every ten minutes.
I've provided a patch that implements conditional get. If the stylesheets haven't been modified since they were last requested, only a 403 Not Modified response is provided.
For this to work, the style_sheet_setup function should no longer be called with time() as the first argument, from the individual styles.php-files in the themes.