Hello,
is there a possibility to define two color schemes and let the system change them automatically according to the current time of day?
For example 8am - 8pm light scheme and from 8pm - 8am a dark one.
Thanks
Not quite what you asked for, but there is night color:
System Settings → Hardware → Display and Monitor → Night Color
There’s an auto-night-mode package for Plasma in the AUR. It’s maintained by @Yochanan. 
pamac build yin-yang-git
Thanks, that is kind of what I was looking for. Unfortunately it does not include the color scheme but only the kde-theme. But its a start, maybe it will follow.
Well, you can select a dark theme and a light theme then. Kvantum-based themes include their own colors. 
kde-themes dont change the color of applications like okular or kate. I do not use kvantum.