System appearance media queries. Now with penguins.

I also added littlefoot footnotes and they should show up correctly in Instapaper now.

Setting media.hardwaremediakeys.enabled in “about:config” allows me to play/pause Overcast and YouTube at last (tested on Firefox 74 and macOS 10.15.4). Switching to Music shows that this works as expected: the latest media played can be controlled by the keys. (via gHacks)