I have been following the switch2osm tutorial explaining how to setup your own tile server:
Everything works fine, however I would like to tweak a few things in order to better understand the rendering curtomization process. As a start I would like to change the color of the country bounderies from the default openstreetmap color to black. Also the bounderies are only displayed at low zoom levels (such as 3) but I would like to enable them at any zoom level.
My understanding is that this kind of styling is controlled by the Mapnik.xml file, which is in turn generated by the
carto project.mml > mapnik.xml command.
How do I modify the project.mml or one of its dependent files in order to achieve my goals?