Since macOS Catalina, there is a OS wide setting to automatically switch from light to dark system theme depending on the hour of the day.

More and more third party software is adopting this functionality and offer the same functionality, so that the whole system behaves more consistent.

Browsers have already implemented this feature and allow web sites to react on it: https://www.howtogeek.com/440920/browsers-are-bringing-automatic-dark-mode-to-websites/

I would love to see Zabbix frontend having an option to make it respect this new feature and also automatically switch theme.