KDE is very feature-rich desktop environment for Linux. However it lacks some of the very essential features for some users like automatic dark mode switch (it’s not present on GNOME either but easily installable with gnome extension) or automatic brightness control for notebooks that have ambient light sensor. So - there are couple of way to enable automatic dark mode on KDE, but non of them is straight forward:

GUI versions

Yin-Yang

can be installed via Flatpak, but it needs to be pulled manually from git and compiled manually.

Koi

Very straight forward if You are using Fedora as there is included Copr repository. Appimage is present as well.

Options with configuration via config file

kshift

Customization solution that uses systemd timers. Installed with pip. Intended for advanced users.

knightadjuster

Single script that allows enable dark mode automatically together with night mode. Positioned as lightweight alternative to Yin-Yang and kshift.