They are cached by default. You can see that if you look at the full url which should have something like &d=1423381380&k=65a747f6bbefc6900a8782d0406186dbf00c70f6. Also, if you inspect the traffic on reload, you should get a 304.
From my understanding, the paint time is not (only) the loading time of the css file, but (also) the rendering process - the actual application of all the css rules to the content of the page.
