The out of box labels on maps have readability issues in a variety of situations and the Zabbix GUI provides no functionality for changing that. The only fix is to create a completely custom theme that applies to all of Zabbix. While you do have documentation on that (https://www.zabbix.com/documentation/6.0/en/manual/web_interface/theming?hl=theme%2Cthemes), that’s a rather tedious way of trying to solve the problem and requires the end user to know enough CSS to try to format as desired. Especially since that has to be manually done on every web server you have. I haven’t personally tested this, but given it requires manual editing of core files, I also assume these would get overwritten during upgrades. The application really should have at least some basic theme management settings in the GUI, especially for maps.