I was using DynamicTheme before but it stopped working with Plasma 6. Didn’t want to wait for someone to implement it so I wrote a little script to check the sunrise and sunset times for my location and change the global theme accordingly. A cron job runs the script every few minutes and the theme changes when applicable.
There are some screenshots in this article:
https://www.phoronix.com/news/Debian-APT-2.9-Released