The configuration variable wgKartographerMapServer already exist and is used in PHP and in JavaScript but not yet in LESS. In specialMap.less ( https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/Kartographer/+/refs/heads/master/styles/specialMap.less ) there is currently hard coded:
background-image: url(https://maps.wikimedia.org/osm-intl/0/0/0.png);
Here the https://maps.wikimedia.org should be replaced by the configuration variable wgKartographerMapServer.