Apologies if this is basic, I've tried to find an answer...
Apparently to add a basemap, I need to change the "style", like here:
style: 'mapbox://styles/mapbox/satellite-v9',
But how do I add this mapbox basemap (Natural Earth II)? https://a.tiles.mapbox.com/v3/mapbox.natural-earth-2.html
... and how to I change basemaps outside of the few that are shown here? https://www.mapbox.com/mapbox-gl-js/example/setstyle/