In that picture, I’m using KDE applications that are flatpaks for Cosmic Desktop on PopOS with a Kvantum theme. I made a longer post here when I was searching for instructions for how to complete this recently.
After my experience, I don’t really know what the best solution is for setting it up. I guess it would be nice if the major platform applications for like KDE were supported for dark mode by default on the DE. I don’t know, it really bothered me though.