Have to admit that I really like it much better than all the Mapnik styles I've tried. Is this https://www.mapbox.com/maps/outdoors/ OSM, or are they using some additional data source?

Looks too good to me to be OSM.

/me curses at the stupid build system.

I am currently toying with the Bluetooth protocol on a Garmin Fenix I got secondhand and I would enjoy something like this to render my little walks