With an account on MapBox and the "classic" version of the MapBox interface, which is an Electron app, I've been able to start customizing the Carto CSS, learning as I go; from that, you can save a .xml file which MapNik can use to build tiles. Starting from the OSM Bright stylesheet, which is nicer than the default carto one, it's quite easy to figure out how to remove and modify things. On Monday, ST and I will sit down and work out a pair of stylesheets, one for pure basics and one with some more modern layers, which we'll make available as separate layers.

I've also written a bash script which processes the XML to create JSON, and then rsyncs the entire site up to ST's own webspace on unix.uvic.ca, so she's now in control of the site.